lab number 3
# 3 Synthesizing and Analyzing a
Coordination Compound of NickelII)
Ion, Ammonia, and Chloride Ion
Prepared by George S. Patterson, Suffolk University, MA
PURPOSE OF THE EXPERIMENT
Synthesize a coordination compound of nickel(II ion, ammonia, and chloride ion. Analyze the compound for mass percent nickel and mass percent ammonia. From these data, determine the empirical formula of the compound, and calculate the percent yield of the synthesis.
BACKGROUND INFORMATION
Two important tasks many chemists perform are the synthesis and analysis of compounds. Synthesis involves not only preparing the compound, but also maximizing the yield of pure product. After isolating the product, the chemist must analyze it to ascertain its chemical composition or formula.
Both tasks require good technique and close attention to what might seem minor procedural details. Therefore, a technically skilled chemist with a good understanding of the purposes of each step in both the synthesis and analysis procedures will get the most accurate results.
In this experiment you will prepare a coordination compound containing nickel(II ion (Nit), ammonia (NHs), and chloride ion (CI).
Then you will determine the empirical formula of the compound. Until you determine the exact formula, we will represent it as Ni(NH3)nC.z, with n representing a small whole number.
Synthesizing Ni(NHs),Clz
You will synthesize Ni(NH3)nCl2 by reacting nickel chloride hexahydrate (NiCl2 • 6H2O) and NH. This reaction is shown in Equation 1.
Ni?+ (aq, green) + 2C1 (aq) + nNH (aq) → Ni(NH3)„Cl(s, bluish purple)
(Eq. 1)
A complication arises because NH3 in aqueous solution is involved in the equilibrium reaction shown in Equation 2.
NH3(aq) + HO(l) = NH4+ (aq) + OH (aq)
(Eq. 2)
Although the equilibrium constant for the reaction in Equation 2 is small,
1.75 × 10-5
, some of the Ni?+ ion can react with hydroxide ion (OH) to
form nickel) hydroxide, Ni(OH, as shown in Equation 3.
Nit (aq, green) + 2OH (aq) → Ni(OH), (s, green)
(Eq. 3)
To the extent that the reaction in Equation 3 occurs, the product formed in Equation 1 will be impure, and the synthesis reaction yield will therefore Water is a convenient solvent for the synthesis reaction because the reactants are water soluble. However, because Ni(NH,),C/½ is also somewhat soluble in water, vou must keep the volume of water vou use in the synthesis to an absolute minimum. Nickel(II chloride hexahydrate is more soluble in hot water than in cold water, so heating the reactants will enable you to dissolve more of this compound in a smaller volume of water.
Unfortunately, the water solubility of NH3 is greatly decreased with increasing temperature. In this case, at temperatures approaching 100°C, NH3 volatilizes before it can react with the NiCh. By maintaining the reaction temperature at 60°C, you will maximize the Ni(NH3)„C12 yield.
Once the reaction is complete, you will cool the reaction mixture to 0°C in an ice-water bath. Because Ni(N),Cl, is less soluble in cold water than in hot water, this step decreases the solubility of the product. You will add cold ethanol to the cold reaction mixture to further reduce the product solubility, because Ni(NH,)„Cl, is insoluble in ethanol.
You will filter the Ni(NH,),Cl, crystals from the cold ethanolic solution and wash them with cold concentrated NH3. This treatment will convert any Ni(OH)2 on the crystals to Ni(NH3),Cl2, as shown in Equation 4.
Ni(OH (s, green) + nNH3(aq) + 2C1 (aq) → Ni(NHa),C(s, bluish purple) + 2OH (aq) (Eq. 4)
Finally, you will dry and weigh the crystals to determine the actual yield of your synthesis.
Analyzing Ni(NH3).Clz
Determining the Mass Percent NH3
Ammonia is a base. Theoretically, you could determine the NH3 content of Ni(NH3)nCl½ by titrating a known mass of the compound with an acid solution of known concentration, called a standardized solution. However, in this case, the procedure gives inaccurate results for two reasons. First, aqueous Ni(NHs)nC solutions slowly evolve NH, so some NH3 may evaporate before the sample is completely titrated. Second, the procedure takes longer than normal because the bonds between NH3 and Nit ion must be broken before NH3 can react with the titrant. This bond breaking is slow and the titration reaction itself is fast, so some of the indicator may react with the acid before all the NH, is released, causing a premature end point.
To minimize NH3 evolution and ensure complete disruption of all NH-Ni?+ bonds, you will use an indirect titration method. You will add a known volume of standard hydrochloric acid solution (HCI) that contains more moles of HCI than there are moles of NH, in the Ni(NH3),Cl sample you are analyzing. The HCI will react with the Ni(NH3),Cl2 sample as shown in Equation 5.
Ni(NH3),C12(aq, bluish purple) + nHCI(aq) → nNHa+ (aq) + Ni?t (aq, green)
+ (n+ 2)CI (aq)
(Eq. 5)
The excess acid and a favorable equilibrium constant drive the reaction in Equation 5 to completion. Then you will titrate the excess HCI with standard sodium hydroxide solution (NaOH) in a process called back titration. The back titration reaction is shown in Equation 6.
HClaq) + NaOH(aq) → NaClaq) + H2O(I)
(Eg. 6)
You will detect the equivalence point of the back titration, the point at which the number of moles of NaOH added is stoichiometrically equivalent to the number of moles of HCl present, by observing an indicator color change. The point at which the indicator changes color is the end point of the titration. In order to select the proper indicator for a particular titration, one that will produce an end point that is close to the equivalence point, you must consider the chemical behavior of the species present in solution at the equivalence point. In this case, if the titration mixture contained only the products shown in Equation 6, the equivalence point would be at pH 7; hence, phenolphthalein, which changes color at pH 8, would be a satisfactory indicator. However, the reaction mixture also contains ammonium ion (NH4 ), which hydrolyzes as shown in Equation 7, causing the titration mixture to be acidic at the equivalence point.
NH4 (aq) + HO(l) = NH3(aq) + HOt (aq)
(Eq. 7)
Therefore, phenolphthalein is not a good indicator for this titration, because the end point would occur after more than an equivalent amount of NaOH had been added. Instead, you will use a mixed indicator solution, composed of bromcresol green and methyl red. Bromcresol green changes from yellow to blue over a pH range of 3.8 to 5.4, and methyl red changes from red to yellow over a pH range of 4.2 to 6.2. The mixed indicator changes from rose to green at pH 5.1 which is the pH at the equivalence point of the titration. Due to the presence of green Nit ion in the mixture, the color change you will see is orange-yellow to green-blue.
From the volume and concentration of HCl solution added, you will
calculate the number of moles of HCI added, using Equation 8.
number of moles of HCI added, mol = (volume of HCI solution added, mL) x 1L/
1000 mL
)(concentration of HC solution, mol L) (Eq. 8)
You will calculate the number of moles of HCI remaining in solution after Ni2+-NH3 separation by substituting into Equation 9, which is a variation of Equation 8, the volume and concentration of NaOH solution required for the back titration.
number of moles of HCI remaining after = (volume of NaOH solution added, mL)
neutralization of NH, mol
1000mL) (concentration of NaOH solution, mol L")
1 mol HUl
1 mol NaOH/
(Eq.9)
The number of moles of NH, in the Ni(NHg),Clz sample is the difference between the number of moles of HCI added and the number of moles of NaOH added, as shown in Equation 10.
number of moles of NHa in = (number of moles of HCl added, mol
Ni(NH3), Cl, sample, mol
-(number of moles of NaOH added, mol)
(Eg. 10)
Finally, you will use Equations 11 and 12 to calculate the mass of NH3 in the sample and the mass percent NH3 in Ni(NH3),Cl2.
mass of NH:
number of moles of
in Ni(NH3),C/2 sample, sample, g= (NH in Ni(NH3),C/2 sample, mol)
17.03 g NH
(Eg. 11)
mass percent NH, in Ni(NH3),Clz, % = (mass of NH3 in Ni(NH3)nCl2sample,g / mass of Ni(NH3)nCl2 sample, g)
(100%) Eq. 12,
Determining the Mass Percent Ni?+ Ion
The [Ni(NH),?+ ion absorbs light in the visible region of the spectrum.
You will take advantage of this light-absorbing property in order to determine the mass percent Ni?+ ion in Ni(NH),Cl. Solutions containing [Ni(NH3)n1?+ ion are colored. The color is produced by those visible wavelengths that are not absorbed. You can determine which wavelengths are absorbed by using a spectrophotometer to measure the absorbance of the solution throughout the visible region of the spectrum. The wavelength at which the species absorbs the most light is called the analytical wavelength (Amax for that species.
Absorbance is directly proportional to the concentration of the absorbing species in solution. This relationship, known as Beer's law, is represented by Equation 13. A is absorbance, s is molar absorptivity, b is the length of the light path through the solution, and c is the molar concentration of the absorbing species.
A = Ebc
(Eg. 13)
Molar absorptivity is a proportionality constant relating absorbance and molar concentration of the absorbing species at the wavelength being measured. The value of s varies with wavelength, reaching a maximum at the analytical wavelength.
Practically speaking, when using a spectrophotometer, we can often read percent transmittance (%) more accurately than we can absorbance.
This is because the %T scale is linear, while the absorbance scale is logarith-mic. Therefore, unless you use a spectrophotometer with a digital absor-bance readout, you should record %T readings and use Equation 14 to convert these readings to their equivalent absorbances.
A = 2.000 - 10g %T
(Eq. 14)
Note that a solution that absorbs none of the light transmits 100% of the light. Therefore, a small absorbance translates to a large %T, and vice versa.
You will prepare a standard [Ni(NH3)„l?+ ion solution by dissolving a known mass of nickel(II sulfate hexahydrate (NiSO, • 6 H2O) in water and adding excess concentrated NH. Then you will dilute the mixture with water to a known volume. The Nit ion in the sample converts to [Ni(NH3),lt ion, as shown in Equation 15.
Ni?+ (aq, green) excess NHs
[Ni(NH3)„2t (aq, bluish purple) (Eq. 15)
You will use your standard [Ni(NH3)n2t ion solution to establish the analytical wavelength (2max) for [Ni(NH)n?+ ion. Then, you will compare the absorbance of your standard [Ni(NH3),1+ ion solution with that of a solution you will prepare from a known mass of the Ni(H3)nClyou synthesized. Because the absorbing species, the [Ni(NH)nk+ ion, is identical in both solutions, & is the same for both solutions. Also, if you use identical cuvettes, the light path length through each sample will be the same. Therefore, using the subscripts to indicate the standard [Ni(NH3)n12+ ion solution and the subscript x to indicate the synthesized Ni(NH3)nCl2 solution, you can write Equations 16 and 17.
As
Ax
Cs, mol L-1=&b=
(Eq. 16)
Cx, mol L-1
Cx, mol L- =
(Ax)(Cx, mol L-')
(Eq. 17)
As
Because one mole of Ni(NH3)„Cl2 contains one mole of Ni?+ ion, the concentration of Ni?+ ion is equal to the concentration of [Ni(NH3)n1+ ion and of Ni(NH3)„Cl2, or Cx. Hence, the relationship of Cx to the number of moles of Ni?+ ion dissolved is shown in Equation 18.
number of moles of Ni?+ ion dissolved, mol
= (Cx, concentration of Ni(NH3), C/2 solution, mol L-')
× total volume of NiNg),Ch solution, mL) 1000 mL.)
(Eg. 18)
From the number of moles of Ni?+ ion dissolved and the mass of compound dissolved, you can calculate the mass percent Nit ion in Ni(NH3)nCl2, using Equations 19 and 20.
mass of Ni? ion in
Ni(NH3),Clz analyzed, e = number of moles of Ni- ion in
Ni(NH3),Cl2 analyzed, mol
(38.69g Ni?tion)
(1 mol Nit ion
(Eg. 19)
mass percent Ni2+ jon — mass of Nit ion in Ni(NH3),C/z analyzed, g) in Ni(NH3), Clz, %
mass of Ni(NH3),Cl2 analyzed, g
(100%)
(Eq. 20)
Note that before you can use your calculated absorbances in Equation 17 you must establish whether or not the cuvettes you use are truly identical, or matched. Matched cuvettes respond identically at max, allowing you to attribute any absorbance differences between solutions in the cuvettes solely to differences in solution concentrations. To determine if your two
cuvettes are matched, you will fill them with distilled or deionized water.
With the water-filled reference cuvette in the cuvette compartment, you will adjust the spectrophotometer to read 100%T. Then you will measure the %T of the water-filled sample cuvette. If the reading for the water-filled sample cuvette is 100%T, the two cuvettes are matched. If the %T is less than 100%T, you must subtract the calculated absorbance for the sample cuvette from the absorbance of both the standard and unknown [Ni(NH3),2+ ion solutions at Amax•
For example, suppose a sample cuvette filled with distilled water has a relative absorbance of 0.005, and the same cuvette filled with standard [Ni(NH),12+ ion solution has an absorbance of 0.315, both at max-The corrected absorbance of the standard [Ni(NH3)n1+ ion solution is 0.315-0.005 = 0.310.
If the water-filled sample cuvette has a negative relative absorbance, it means that the sample cuvette is more transparent than the reference cuvette. To compensate for this, you should replace the water-filled sample cuvette in the spectrophotometer compartment and adjust the spectrophotometer to obtain a meter reading of 100%T. Then you should insert the water-filled reference cuvette and determine its relative absorbance.
You must add the absorbance of the water-filled reference cuvette to the absorbance of the standard and unknown [Ni(NH)n?+ ion solutions at Amax-
Determining the Empirical Formula of the Coordination
Compound
The coordination compound you will synthesize is composed of Ni?+ ion, NH3, and CI ion. Therefore, you can calculate the mass percent Cl ion in the compound by subtracting the mass percents of Ni?+ ion and NH, from 100%, as shown in Equation 21.
mass percent CI ion in Ni(NH3), Cl, % = (100%) - (mass percent Nit ion in Ni(NH3), Cl2, %)
+ (mass percent NH, in Ni(NH3), Clz, %)
(Eq. 21)
Assume you have 100g of the compound. Then, the mass percent of each component would be equivalent to the number of grams of each component in the 100g sample. Thus, based on the mass percents of the components, you can calculate the empirical formula of the compound, using Equations 22-24.
number of moles of Ni?tion, mol = (moss of Ni't ion, g)
1 mol Ni?+ ion
58.69 g Nit ion
(Eq. 22)
number of moles of NH, mol = (mass of NH3, g
(Eq. 23)
1 mol NH3/ 17.04g NH3,
number of moles of Clion, mol = (mass of Clion, g)
1 mol Clion)
35.45 g Cl ion
(Eq. 24)
The empirical formula of a compound is the simplest whole-number ratio among the numbers of moles of the components, that is Ni,(NHg),Cly
where x, n, and y are whole numbers. Often, we can find the values of x, n, and y by dividing each number of moles obtained using Equations 22-24 by the smallest number of moles. The results of these divisions, rounded to the nearest whole numbers, will give the empirical formula of the product you synthesized
From the number of moles of NiCl2 • 6 H2O used in your synthesis, you will calculate the theoretical yield of product in moles, using Equation 25.
number of moles of NiC • 6H2O used, mol = number of moles of product, mol (Eq. 25)
From the mass of product obtained and the molar mass of the product, you will calculate the actual yield of product in moles, using Equation 26.
actual yield of product, mol mass of product obtained, g
molar mass of product, g mol 1
(Eg. 26)
Finally, you will calculate the percent yield of your synthesis, using
Equation 27.
actual yield of product, mol
percent yield of product, % = theoretical yield of product, mol) (100%) (Eq. 27)
PROCEDURE
CHEMICAL ALERT
acetone flammable and irritant concentrated ammonia-toxic, corrosive, and lachrymator 95% ethanol-toxic and flammable
0.25M hydrochloric acid solution-toxic and corrosive nickel(Il) chloride hexahydratetoxic and suspected carcinogen nickel(I) sulfate hexahydrate-toxic, irritant, oxidant, and suspected carcinogen
0.10M sodium hydroxide solution-toxic and corrosive
CAUTION
Wear departmentally approved eye protection while doing this experiment.
You are strongly urged to wear latex or vinyl gloves while performing all parts
of this experiment.
I. Synthesizing Ni(NH 3) ,Cl2
© 1994 Cengage Learning
CAUTION
Nickel(Il) chloride hexahydrate is toxic and a suspected carcinogen.
Prevent eye, skin, and clothing contact. Avoid inhaling dust and ingesting the compound.
if you should spill any of the solid, immediately notify your laboratory
instructor.
After handling the solid, immediately wash your hands and face with soap or
detergent before proceeding with the experiment.
2
NOTE: Your laboratory instructor will describe and demonstrate a satisfactory method for weighing and transferring a solid sample and will inform you as to the number of significant digits to the right of the decimal point to record masses.
Prepare a warm-water bath. Half fill a 600-mL or larger beaker with tap water. Attach a large ring support to a ring stand. Place the beaker through the ring. Adjust the ring so that the beaker is stablilized while sitting on a hot plate, as shown in Figure 1.
Monitor the water temperature with a thermometer that you have carefully inserted through a one-hole stopper. Clamp the stopper with the inserted thermometer to the ring stand, making sure that the thermometer extends into the water but does not touch the side or bottom of the beaker.
Heat the beaker and its contents until the water temperature reaches 50°C.
Adjust the hot plate setting so that the water temperature remains between 50 and 60°C.
Weigh a piece of weighing paper and record the mass on Data Sheet 1.
Weigh 8.0 g of NiCh • 6H,O on the weighed piece of weighing paper.
Record the mass of the solid and paper on Data Sheet 1. Transfer the solid to a clean 125-mL Erlenmeyer flask.
Add 10 mL of distilled or deionized water to the NiC • 6H2O in the flask. Place the flask in the 60 °C water bath and clamp the flask in position, as shown in Figure 2. Stir the mixture in the flask with a clean 125- mm glass stirring rod until the NiC • 6H2O has dissolved. Loosen the clamp on the ring stand, and while holding the end of the clamp, remove the flask from the bath. Attach the clamp to another ring stand, and let the flask and contents cool in air for 1-2 min.
43
CAUTION
Concentrated NH3 solution is toxic, corrosive, and a lachrymator. Prevent eye, skin, and clothing contract. Permanent fogging of soft contact lenses may result from NH3 vapors. Avoid inhaling vapors and ingesting the solution. Unless your laboratory instructor tells you otherwise, restrict all work with this reagent to a fume hood.
If you spill any NH3 solution on yourself, immediately rinse with a large amount of running water. If you spill any NH3 solution on the laboratory bench, add water and wipe it up immediately with a damp paper towel. Immediately notify your laboratory instructor of any NH3 solution spills. Dispose of NH- soaked paper towels as directed by your laboratory instructor.
You should cover the top of the reaction flask and any other containers of concentrated NH3 with a wet paper towel, in order to limit your and others' exposure to NH3 vapors. The moisture in the towel will absorb NHa fumes, forming aqueous NH3.
Slowly, with stirring, add 25 mL of concentrated NH3 solution to the NiCh2 • 6H2O solution in the flask. Cover the top of the flask with a wet paper towel. After adding the concentrated NH3 solution, suspend the flask in the warm-water bath by clamping it to the ring stand. Make sure the water temperature is between 50 and 60°C. Leave the flask in the bath for 15 min. During this time, periodically swirl the mixture.
NOTE: For the filtering apparatus described below, your laboratory instructor will inform you whether you are to place a trap between your filter flask and the water aspirator and whether you are to place a Büchner funnel in a rubber FilterVac, a bored rubber stopper, or a rubber filtering adapter.
Do not turn off the aspirator before removing the tubing from the side arm of the
filter flask.
While the above reaction proceeds, assemble your filtering apparatus, as shown in Figure 3. Clamp a 250-mL taped filtering flask to your other ring stand, and place a Büchner funnel in the flask.
Place a flat circle of filter paper in the funnel. Wet the paper with 1-2 mL of distilled water. Attach a piece of pressure tubing to the flask and to the water aspirator. Turn on the aspirator to draw the water in the funnel into the flask and to snugly seal the filter paper to the funnel. Disconnect the tubing from the filter flask and turn off the aspirator. Remove the funnel, and pour the water from the filtering flask into the drain. Reassemble the vacuum filtration assembly.
CAUTION
95% Ethyl alcohol is flammable and should not be exposed to an open flame in the laboratory.
Prepare an ice-water bath in another 600-mL beaker by adding 150 mL of water and several pieces of ice to the beaker. Transfer 20 mL of concentrated NH, solution into a labeled, 18 × 150-mm test tube. Stopper the test tube with a No. 2 solid rubber stopper. Place the test tube in the ice-water
bath. Obtain 60 mL of 95% ethanol in a labeled 100-mL beaker. Place the beaker and its contents in the ice-water bath.
Assemble a second ice-water bath in a 600-mL beaker. After the reaction in the warm-water bath has proceeded for 15 min, unclamp the flask from the ring stand. Carefully clamp the reaction flask on another ring stand so that the flask is suspended in the second ice water bath. Remove the damp paper towel covering the mouth of the flask. While holding the flask, loosen the clamp and swirl the reaction mixture for 5 min while it is cooling. Add 10 mL of ice-cold 95% ethanol to the flask and stir. Remove the reaction flask from the ice-water bath. Wipe any water off the bottom of the flask using a paper towel.
After reassembling the vacuum filtration apparatus, turn on the aspirator. Slowly pour the liquid-solid mixture from the flask into the Büchner funnel, as follows. Decant as much supernatant liquid as possible into the funnel. Use a stirring rod to guide the liquid from the flask onto the filter paper, in order to prevent splashing and product loss. Then use a rubber policeman attached to another glass stirring rod to help transfer the solid from the flask into the funnel. When all the liquid has been drawn through the funnel, disconnect the tubing from the filter flask arm, and turn off the aspirator.
Rinse any remaining solid down the inside wall of the reaction flask using 5 mL of ice-cold, concentrated NH3 solution from your test tube. Swirl the solid and rinse solution mixture, and quickly pour the mixture into the Büchner funnel. Reattach the tubing to the filter flask, and turn on the aspirator to draw the rinse solution through the funnel. Then disconnect the tubing and turn off the aspirator.
In the same manner, rinse any remaining solid from the flask using two additional 5-mL portions of the cold, concentrated NH, solution. After pouring any remaining concentrated NH, rinse solution over the solid in the funnel, reattach the tubing, and turn on the aspirator.
Dry the solid by drawing air through the solid for 3-5 min. Disconnect the aspirator, and turn off the aspirator. Break up the solid with a spatula,being careful not to tear the filter paper. Pour 15 mL of cold 95% ethanal over the solid, reattach the tubing, and turn on the aspirator. Repeat the ethanol washing two more times, using 15 mL of 95% ethanol each time.
Make sure to disconnect the tubing, turn off the aspirator, and carefully break up the solid before each ethanol wash. When you have finished the third wash and have dried the solid, disconnect the tubing and turn off the aspirator.
CAUTION
Acetone is flammable and should not be exposed to an open flame in the laboratory.
Pour 15 mL of acetone over the solid in the funnel. Break up the solid with a spatula to expose all its surfaces to the acetone. Reattach the tubing and turn on the aspirator to draw the acetone through the funnel. To completely dry the solid, leave the aspirator on and draw air through the solid for 10-15 min.
NOTE: Your laboratory instructor will tell you whether or not your solid is dry enough to be weighed. If it is not, you should further air dry your solid.
Determine the mass of a capped weighing bottle labeled with your name. Record this mass on Data Sheet 1. After the solid is completely dry, add it to the weighing bottle. Close the bottle tightly, and determine the mass of the bottle and solid. Record this mass on your Data Sheet. Your laboratory instructor will tell you where to store your weighing bottle and its contents.
Transfer the solution in the filter flask into the container provided by your laboratory instructor and labeled "Discarded NiCl/ Ethanol/ Acetone Solution Mixture." Rinse the filter flask and Büchner funnel once with 20 mL of tap water. Transfer the rinse to the same discard container.
II. Determining the Mass Percent NHa in Ni(NH),Cl½
by Titration
NOTE: Your laboratory instructor will demonstrate and describe proper techniques for the quantitative use of a buret and pipet.
Record all buret readings to the nearest 0.02 mL.
CAUTION
0.1/NaOH solution is corrosive and should be handled with care. Prevent eye, skin, and clothing contact. Avoid ingesting the solution.
Obtain 200 mL of standardized NaOH solution in a clean, dry, stoppered
250-mL Erlenmeyer flask. Record the exact NaOH solution concentration on Data Sheet 2. Number four clean 125-mL Erlenmeyer flasks #1-4.
Clean your 50-mL buret if necessary. Rinse the clean buret with distilled water and then with 5 mL of the NaOH solution. Transfer the rinses into the container provided by your laboratory instructor and labeled "Discarded Ni(NH),C//HCI/NaOH Solution Mixture."
Fill the buret to or just below the 0-mL mark with NaOH solution. Drain 1-2 mL of the NaOH solution from the buret into the discard beaker, both to fill the buret tip and to make sure that there are no air bubbles in the buret tip. Record your initial buret reading on Data Sheet 2 in the column headed
"determination 1."
Using an analytical balance, weigh out from your weighing bottle four 0.12 to 0.14 g samples of the Ni(NHs),Ch you prepared in Part I. Record the mass of each sample to the nearest 0.1 mg on Data Sheet 2. Carefully transfer each sample to a different one of your four numbered Erlenmeyer flasks.
Make sure to note on flask labels which sample you transfer to which flask.
CAUTION
0.25M HCI solution is a corrosive solution that can cause skin irritation.
Prevent eye, skin, and clothing contact.
Obtain 125 mL HCl solution. Record the exact concentration of this solution on Data Sheet 2. Using a clean 25-mL volumetric pipet, transfer 25.00 mL of approximately 0.25M HCI solution to each numbered flask. Using your
100-mL graduated cylinder, add 10 mL of distilled water to each flask. Then add 2 or 3 drops of the bromcresol green-methyl red mixed indicator solution to each flask.
Titrate the sample in flask #1 to determine the approximate volume of NaOH solution required and to observe the end point. While swirling the flask with one hand, control the buret stopcock with your other hand. When the orange-yellow color of the titration mixture first begins to change, continue to add NaOH solution dropwise, just until the mixture turns green-blue. Record the final buret volume in the column for determination 1 on Data Sheet 2.
NOTE: Remember that a larger mass of Ni(NH3)nCl2 will liberate more NH3 and consume more HCIthan will a smaller mass of Ni(NH3)nCl. Thus, the amount of excess HI will be greater for a smaller Ni(NHs),Cl sample than for a larger one.
Using the mass of Ni(NHg),C/2 in flask #1 and volume of NaOH solution used for flask #1 as a guide, titrate the solutions in flasks #2, #3, and
Data Sheet 2.
#4. Record initial and final buret readings in the appropriate columns on
Transfer any NaOH solution remaining in your buret as well as the titration solutions to the "Discarded Ni(NH3),Cl/HCI/NaOH Solution Mixture" container. Rinse the buret, pipet, graduated cylinder, and Erlenmeyer flasks twice each, first with tap water, and then with distilled water. Transfer all rinses to the discard container.
Determining the Mass cent Ni2+ lon in Ha)„Cla by
trophotometry
CAUTION
Nickel(Il) sulfate hexahydrate is toxic and a suspected carcinogen. Prevent eye, skin, and clothing contact. Avoid inhaling dust and ingesting the compound.
If you should spill any of the solid, immediately notify your laboratory
instructor. On the frosted or white circle on a clean, dry 100-mL beaker, write "std" to indicate the NiSO, • 6 HO standard sample solution. Write "unk" on a second clean, dry 100-mL beaker, which you will use for your Ni(NH3),Clz sample solution with unknown %Ni?+ ion. Clean two 50-mL volumetric flasks and stoppers. Label one flask "std" and the other flask "unk."
Using an analytical balance, weigh on a weighing paper a 0.20 to 0.40 g NiSO4 • 6 H2O sample. Transfer the sample to the "std" beaker. Record the mass of NiSO, • 6 H2O to the nearest 0.1 mg on Data Sheet 3.
Using an analytical balance, weigh on a weighing paper a 0.30 to 0.40 g sample of your Ni(N3)„C2. Transfer the sample to the "unk" beaker.
Record the mass of Ni(NH3),Cl2 to the nearest 0.1 mg on Data Sheet 3.
Using a graduated cylinder, add 20 mL of distilled water to both samples. Note and record on Data Sheet 3 the color and appearance of each solution. Using separate glass stirring rods, stir each mixture until most of the solid has dissolved. Leave the rods in the beakers to avoid losing any solution adhering to the rods.
NOTE: Your laboratory instructor will demonstrate the use of volumetric flasks. Prepare the following solutions and transfer them to volumetric flasks in a fume hood.
Measure out 10 mL of concentrated NH3 solution in a 10-mL graduated cylinder, which need not be dry. Add the NH3 solution to the solution in the
"std" beaker. Then measure another 10 mL of concentrated NH3 solution, and add it to the solution in the "unk" beaker. Stir each mixture until no solid remains. Note and record on Data Sheet 3 the color and appearance of each solution.
Using a short-stem funnel, transfer the "std" solution into the "std"
50-mL volumetric flask. Rinse the beaker and rod with a minimum amount of distilled water from a wash bottle, and pour the rinses into the "std" volumetric flask. Rinse the beaker two more times, using distilled water, but do not allow the volume of solution in the volumetric flask to exceed 50mL.
Add distilled water to the solution in the flask until the solution level coincides with the junction of the neck and body of the flask. Stopper the flask. Firmly holding the stopper in place, invert the flask 10 times to thoroughly mix the solution. Then fill the flask exactly to the etched mark by adding distilled water from a disposable pipet or medicine dropper.
Stopper the flask. Thoroughly mix the solution by inverting the flask at least 25 times, while holding the stopper firmly in place.
Follow the same procedure to transfer your "unk" solution to the
"unk" 50-mL volumetric flask. Dilute the "unk" solution to the etched mark, stopper the flask, and thoroughly mix.
Obtain two spectrophotometer cuvettes, and place them in a dry beaker or test tube rack. Clean the cuvettes and rinse them with distilled water.
Using a pencil, write "R" on the frosted glass circle near the top of one cuvette for "reference." Mark the other cuvette "S" for "sample." Fill the reference cuvette about three-quarters full with distilled water. Rinse the sample cuvette three times, using a 1-2 mL of your "sta" solution each time. Dispose of the rinses into the container provided by your laboratory instructor and labeled "Discarded NiC/NiSO/NH, Solutions." Fill the sample cuvette about three-quarters full with "std" solution. Place the filled cuvettes in a beaker or test tube rack.Set the wavelength on the spectrophotometer to 620 nm. With the cuvette compartment empty and its cover closed, adjust the left-hand knob until the meter reads 0%T. To avoid a parallax error, look directly down at the needle so that you cannot see its reflection in the mirror behind the needle.
Using a tissue, wipe off the water-filled reference cuvette, and insert it into the cuvette compartment. Close the cover. Turn the right-hand knob until the meter reads 100%T. Transfer the cuvette to the beaker or test tube rack. Wipe the filled sample cuvette, and insert it into the compartment.
Read the %T. Record this %T on Data Sheet 3 as the %T of the INi(NH3),12+ ion standard solution at 620 nm. Transfer the sample cuvette to the beaker or test tube rack and close the cover.
Adjust the wavelength to 600 nm. Following the above procedure, first with the reference cuvette and then the sample cuvette, obtain and record the %T for the standard solution at 600 nm. Repeat the procedure at wavelength intervals of 20 nm down to 540 nm. From among your five %T readings, determine the approximate max for the [Ni(NH3)n1t ion and record it on Data Sheet 3. To more precisely establish imax for the [Ni(NH3),12t ion, measure the %T of the standard solution at wavelengths 10 nm less and 10 nm greater than the wavelength you estimate as Amax-Record these additional %T measurements on Data Sheet 3. Select the 2max for the [Ni(NH3)nI?t ion and record it on Data Sheet 3.
Empty the sample cuvette into the "Discarded NiClz/NiSO4/NH3
Solutions" container. Rinse the cuvette with distilled water, and then rinse it three times with your "unk" solution, using 1 mL of solution each time.
Transfer all rinses to the discard container.
Fill the cuvette about three-quarters full with "unk" solution. Set the spectrophotometer at the analytical wavelength. Check the 0%T setting with the cuvette compartment empty and its cover closed. Check the 100% T setting using the water-filled reference cuvette. Determine the %T of the unknown solution at 2max. Record this %T on Data Sheet 3. Transfer the solutions in your cuvettes to the discard container. Rinse and wash the cuvettes and add any rinses and washings to the discard container.
Finally, check to see whether or not the reference and sample cuvettes are matched at 2max. To do so, rinse and fill the sample cuvette three-quarters full with distilled water. With the wavelength set at 2max and the cuvette compartment empty and closed, adjust the right-hand knob until the meter reads 0% T. Then place the reference cuvette filled with distilled water in the cuvette compartment. Adjust the right-hand knob so that the meter reads 100%T. Finally, measure the %T of the water-filled sample cuvette. Record this %T on Data Sheet 3. If the meter needle goes off the scale to the right of 100%T, the sample cuvette is more transparent than the reference cuvette at Amax. To correct for this difference, replace the water-filled sample cuvette in the compartment and adjust the meter to read 100%T. Then reinsert the water-filled reference cuvette and measure its %T. Record this %T on Data Sheet 3.
Transfer the solutions in your volumetric flasks into the appropriate discard container. Rinse the volumetric flasks twice with 10 mL of tap watereach time and twice with 10 mL of distilled water each time. Transfer the rinses into the appropriate discard container. Allow the flasks to drain.
Empty the cuvettes into the drain and allow them to dry.
CAUTION
Wash your hands thoroughly with soap or detergent before leaving the laboratory.
CALCULATIONS
1. Synthesizing Ni(NHg),Cl2
Il. Determining the Mass Percent Ng in Ni(NH3),Clz
by Titration
III. Determining the Mass Percent Ni?+ lon in
Spectrophotometry
IV. Determining the Empirical Formula and Percent Yield of Ni(NHa),Cl2
Record the results of the following calculation on Data Sheet 1.
1. Calculate the mass of NiC • 6 H2O used in the synthesis.
2. Calculate the mass of synthesized Ni(NH3),Clz.
Do the following calculations for each determination and record the results on Data Sheet 2.
3. Calculate the mass of Ni(NH)nCl sample titrated.
4. Calculate the number of moles of HCI added to the Ni(NH),Cl solution.
5. Calculate the number of moles of NaOH required for titration.
6. Calculate the number of moles of NH3 present in the Ni(NH3)nCl2 sample.
7. Calculate the mass percent NH3 in Ni(NH3)nClz.
8. Calculate the mean mass percent NH3 in Ni(NH3)nCl2.
Do the following calculations for each determination and record the results on Data Sheet 3.
9. Calculate the mass of NiSO4 • 6H2O in the standard solution.
10. Calculate the mass of Ni(NH3)„Cl½ in your unknown solution.
11. Convert all %T readings to A.
12. Calculate the corrected A, for the NiSO, solution.
13. Calculate the corrected Ax for the Ni(NH3)nCl2 solution.
14. Calculate the mass percent Nit ion in Ni(NH3),Cl2.
Record the results of these calculations on Data Sheet 4.
15. Calculate the mass percent Clion in Ni(NH3)„Cl2.
16. Determine the empirical formula of Ni(NH3)nCl2.
17. Calculate the molar mass of Ni(NH3)nCl2.
18. Calculate the number of moles of synthesized Ni(NH3),Cl2.
19. Calculate the number of moles of NiC • H2O used in the synthesis.
20. Calculate the percent yield of your synthesized Ni(NH3),Cl2 based on the mass of NiCl • 6H2O used in the synthesis.