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What is the purpose of Experiment 2 ?
Determine the concentration of an unknown Al³⁺ solution using gravimetric analysis by precipitating aluminum as Al(hq)₃, filtering, drying, and weighing the solid to quantify Al³⁺ with stoichiometry.
What is gravimetric analysis in Experiment 2?
A type of quantitative analysis that relies on mass measurement to determine the amount of an analyte by converting the soluble analyte into an insoluble product to be collected, filtered, and weighed
What is the purpose of precipitation in Experiment 2?
Convert the analyte (Al³⁺) into an insoluble solid (Al(hq)₃) that is large enough to be collected, weighed, and filtered
What is a ligand? (Experiment 2)
Molecule or anion that can donate an electron pair to the metal ion (Lewis base)
What is a coordination compound? (Experiment 2)
Consists of a metal ion (Lewis Acid) and one or more ligand (Lewis Base)
What is a coordination number? (Experiment 2)
The number of atoms bound to the metal center
What is the coordination number of Al3+?
6 because three hq ligands each donate two electron pairs
What is the (bidentate) ligand in experiment 2?
8-hydroxyquinoline (hq), where 1 oxygen and 1 nitrogen bind to the metal ion
Why is Al(hq)₃ used in experiment 2?
It is an insoluble complex with a high molar mass, allowing for precise gravimetric analysis of Al³⁺ concentration
Why are crucibles dried at 100–120°C in Experiment 2?
Remove moisture that can affect the mass reading
How does pH affect the binding of 8-hydroxyquinoline (hq) to Al³⁺? (Experiment 2)
Below pH 5: hq is protonated (Hhq⁺) and soluble but cannot bind to Al³⁺.
Between pH 5-10: hq is neutral and somewhat soluble but can bind to Al³⁺.
Above pH 10: hq is deprotonated and highly soluble, but results in undesired side products like Al2O3, Al(OH)₃, and Al(OH)₄⁻
Why is the reaction carried out near pH 4 in Experiment 2?
Balances solubility and reactivity for effective precipitation and avoiding unwanted aluminum side products.
What happens if too much hq (ligand) is added in Experiment 2?
Contaminates the precipitate, leading to overestimated Al³⁺ concentration measurements
Why is initial heating of Al³⁺ solution before adding Hq important in Experiment 2?
Prevent premature precipitation and to increase solubility to better control the reaction
What is the buffer used in Experiment 2 and why?
Ammonium acetate (NH4+CH3COO-) is added to raise pH ~4 for optimal Al(hq)₃ precipitation
Why is the precipitate left undisturbed overnight in Experiment 2?
Slow precipitation results in larger and purer crystals to improve accuracy
What is the purpose of vacuum filtration in Experiment 2?
Effectively separate solid Al(hq)₃ from the liquid
What is the purpose of A desiccator in general?
Designed to store objects in a dry atmosphere without moisture
What is the difference between hydrophobic and hydrophilic molecules? (Experiment 2)?
Hydrophobic molecules repel water and tend to be nonpolar, while hydrophilic molecules interact well with water and are usually polar.
Is 8-hydroxyquinoline (hq) hydrophobic or hydrophilic? (Experiment 2)?
Depends on pH.
In its neutral form (pH 5-10), it is hydrophobic and precipitates
In its protonated (pH <5) or deprotonated (pH >10) forms, it is more hydrophilic
Why does Al(hq)₃ precipitate in Experiment 2?
Neutral form of hq is poorly soluble in water and binds to Al³, forming a hydrophobic complex that precipitates out of solution
How is the unknown concentration of Al³⁺ determined? (Exp 2)
Weighing the dried Al(hq)₃ precipitate and using stoichiometry to calculate the moles of Al³⁺ in the original solution
-Professor’s Example (Experiment 2):
What are the limiting and excess reagents? Select one.
A) hq⁻ is the limiting reagent, and Al³⁺ is the excess reagent.
B) Al³⁺ is both the limiting reagent and the excess reagent.
C) hq⁻ is both the limiting reagent and the excess reagent.
D) Both Al³⁺ and hq⁻ are limiting reagents; there are no excess reagents.
E) Al³⁺ is the limiting reagent, and hq⁻ is the excess reagent.
Answer: E) Al³⁺ is the limiting reagent, and hq⁻ is the excess reagent.
-Professor’s Example (Experiment 2):
What is the purpose of NH₄CH₃COO? Select one.
A) Increase the pH to at least 10.
B) Allow the pH to fluctuate between 2 and 10, as needed.
C) Maintain a constant pH around 7.
D) Maintain a constant pH around 5.
E) Decrease the pH to at least 2.
Answer: D) Maintain a constant pH around 5.
-Professor’s Example (Experiment 2):
You were in a rush and allowed the precipitate to form for only 15 minutes, not the required minimum of one hour. How, if any, will the reported concentration of Al³⁺ be affected by this reduction in precipitation time? Select one.
A) The reported concentration of Al³⁺ will be artificially low because the pH will be incorrect.
B) The reported concentration of Al³⁺ will be artificially high or low, but you cannot determine the direction.
C) The reported concentration of Al³⁺ will be artificially high because some of the starting material will not have reacted.
D) The reported concentration of Al³⁺ will be artificially low because some of the starting material will not have reacted.
E) The reported concentration of Al³⁺ will be artificially high because the pH will be incorrect.
Answer: D
Less time = unable to complete full reaction = less mass = lower concentration (b/c g/mol)
-Professor’s Example (Experiment 2):
Select the two true statements about gravimetric analysis.
A) Gravimetric analysis is defined as quantitative analysis via mass.
B) Gravimetric analysis is defined as quantitative analysis via volume.
C) In the context of this experiment, gravimetric analysis involves conversion of a soluble analyte to an insoluble product.
D) In the context of this experiment, gravimetric analysis involves conversion of an insoluble analyte to a soluble product.
E) In the context of this experiment, gravimetric analysis involves conversion of a soluble analyte to a soluble product.
Answer: A & C
-Professor’s Example (Experiment 2):
What are the starting material and the desired yellow product? Select one.
A) Aqueous Al(NO₃)₃ is the starting material, Al(hq)₃ is the yellow product.
B) Aqueous Al(Hhq)₃ is the starting material, Al(hq)₃ is the yellow product.
C) Aqueous Al(NO₃)₃ is the starting material, [Al(OH)₄]⁻ is the yellow product.
D) Aqueous Al(hq)₃ is the starting material, Al(Hhq)₃ is the yellow product.
E) Aqueous Al(hq)₃ is the starting material, Al(H₂O)₆³⁺ is the yellow product.
Answer: A) Aqueous Al(NO₃)₃ is the starting material, Al(hq)₃ is the yellow product.
What is the purpose of Experiment 3?
Identify an unknown weak organic acid by determining its molecular weight and pKa through acid-base titration using standardized NaOH and volumetric analysis.
What is volumetric analysis in Experiment 3?
A type of chemical analysis that determines the unknown analyte’s concentration by measuring the volume of titrant needed to reach the equivalence point
What is the difference between Titrant and Analyte? (Experiment 3)
Titrant is a solution with known properties
Analyte is a solution with unknown properties of interest
Define standardization in Experiment 3
determining the exact concentration of a solution (NaOH) by titrating with primary standard KHP.
What is an indicator? (Experiment 3)
A dye that causes a visible color change at a certain pH
Choose an indicator with a pKa that is as close as possible to the expected pH of the analyte at the equivalence point
Why do we rinse the buret with NaOH before filling it? (Experiment 3)
Ensure the concentration of NaOH is not diluted by residual water
What is the equivalence point in a titration? (Experiment 3)
Moles of acid = moles of base (neutralization)
What is the difference between the equivalence point and the endpoint? (Experiment 3)
Equivalence point is theoretical (mathematically determined) while the endpoint is when the indicator changes color
Why do we use phenolphthalein as the indicator? (Experiment 3)
Why do we assume a 1:1 reaction between the unknown acid and NaOH in Experiment 3?
Because weak organic acids typically donate one proton per molecule in acid-base titration
The Henderson-Hasselbalch Equation (Experiment 3):
At halfway point, pH = pKa
What does the acid ionization constant (Ka) tell us? (Experiment 3):
Strength of the acid; Smaller Ka = Weaker Acid
How do we determine the pKa from the titration curve in experiment 3?
Finding the pH at the half-equivalence point, where half of the acid has been neutralized
How did we identify the unknown acid in the end? (Experiment 3)
Lowest % error of molar mass and pKa on Excel
Why and how do we find the 1/3, 1/2, and 2/3 equivalence point
To determine the pKa of the unknown acid.
We divide our equivalence point by 1/3, 1/2, or 2/3. Plug that value you into our linear fit equation. Then use the Henderson-Hasselbach formula.
How and why was our pH meter calibrated in Experiment 3?
Using buffers with pH 4.00 and 7.00 to prevent reading shifts
How does a pH meter work in Experiment 3?
It has a glass electrode that detects H₃O⁺ ion concentration, converting it into a voltage signal corresponding to pH.
Why do we store standardized NaOH in a tightly sealed bottle in Experiment 3?
To prevent CO₂ absorption, which forms carbonic acid and decreases the NaOH concentration
Why is KHP dried before weighing in Experiment 3?
To remove moisture that can affect the mass reading
What is a titration in Experiment 3?
Technique of gradually adding titrant to analyte to determine its concentration
Why is KHP used as the primary standard in Experiment 3?
It is a highly pure, high molar mass, stable, and non-hygroscopic solid to ensure accurate and precise standardization of NaOH solution
What is the indicator in Experiment 3 and why?
Phenolphthalein because it changes color around pH 8-9
Why perform a rough (coarse) titration before actual trials in Experiment 3?
To estimate the volume of NaOH needed for better precision.
Why is KCl salt added in experiment 3?
Improves conductivity and stabilizes pH readings
Professor’s Example (Experiment 3):
What situation, if any, will result in an artificially low molecular weight? Select one.
A) Too little water is added to the unknown acid prior to starting the titration.
B) During the calculation, you accidentally use a value of 0.010 M for [NaOH] instead of the actual concentration of 0.100 M.
C) You accidentally record the mass of the unknown as 0.3300 g instead of the actual mass of 0.3003 g.
D) You rely on a dark pink solution to indicate that the analyte has been neutralized. The volume of base that corresponds to this dark pink solution is used in your calculations.
E) None of these situations will cause the calculated molecular weight to be artificially low.
Answer: D
A) water does not affect the reaction
B) Mol is lower → Molecular weight increased (from g/mol)
C) Mass increase → Molecular weight increased (from g/mol)
D) Vol x M (mol/L) = mol; Mol increase = molecular weight decrease (from g/mol)
Professor’s Example (Experiment 3):
During lecture, we calculated the pH at the equivalence point of a weak acid (KHP) titrated with a strong base (NaOH) in order to select the correct indicator. What information was not needed to select the appropriate indicator? Select one.
A) pH at which the indicator dye undergoes a color change
B) pKₐ of the weak acid
C) Concentration of NaOH
D) Initial pH of the weak acid (before addition of titrant)
E) Initial concentration of the weak acid
D) Initial pH of the weak acid (before addition of titrant)
-Professor’s Example (Experiment 3):
During a coarse titration, you placed 200 mg of unknown acid in 80 mL water, and dispensed 4 mL of NaOH to this acid solution (analyte) to reach the endpoint. What should be the mass and volume of the analyte so it takes 16 mL of NaOH to reach the endpoint? Select one.
A) 200 mg in 320 mL water
B) 50 mg in 80 mL water
C) 200 mg in 20 mL water
D) 800 mg in 80 mL water
E) 400 mg in 80 mL water
Answer: D
Reasoning: Water in this scenario is extra.
4 mL * x = 16 mL → x = 4
Therfore, 200 mg of unknown acid x 4 = 800 mg of unknown acid
Professor’s Example (Experiment 3):
What situation, if any, will definitely lead to an artificially low pKₐ value? Select one.
A) You use a ruler instead of the first derivative to analyze the titration curve.
B) [NaOH] is accidentally recorded as 0.101 M instead of the actual 0.111 M.
C) You accidentally create a 0.040 M solution of unknown acid instead of 0.050 M.
D) You accidentally record the equivalence point as 8.08 mL instead of the actual 8.80 mL.
E) None of these situations will cause the pKₐ value to be artificially low.
Answer: D
What is the purpose of Experiment 4?
Synthesize and analyze an Fe(III)-oxalate complex by determining its empirical formula and iron content using spectrophotometry
What is being synthesized in experiment 4?
A Fe(III)-oxalate complex in the form of Kw[Fex(C2O4)y]∙zH2O, where oxalate is the ligand to Fe³⁺
Why is oxalate used as a ligand? (Experiment 4)
Oxalate is a bidentate ligand - it donates two lone pairs to Fe³⁺ to be more stable
Why do we use hydrogen peroxide (H₂O₂) in this synthesis? (Experiment 4)
To oxidize Fe²⁺ to Fe³⁺, allowing the formation of Fe(III)-oxalate.
Why is ethanol (EtOH) used during crystallization of Experiment 4?
Fe(III)-oxalate is less soluble in ethanol than in water, promoting crystallization and purification.
How is the empirical formula of the Fe(III)-oxalate determined in Experiment 4?
Using the given % mass of oxalate (C₂O₄²⁻) and experimentally determined % mass of Fe³⁺ to find K with net charge and remaining % mass for H2O
How do you determine the percent yield of the synthesis in Experiment 4?
Comparing the actual mass vs theoretical mass based on the limiting reagent.
Why is recrystallization performed in Experiment 4?
To obtain purer Fe(III)-oxalate crystals by dissolving impure solid and allowing slow formation of well-ordered crystals
How can you tell if the crystals are of good quality in Experiment 4?
Good crystals are plate/block-like and bad crystals are powdery or contaminated with clear crystals
Why is vacuum filtration used in Experiment 4?
To efficiently separate and dry the Fe(III)-oxalate crystals from the solution.
Why is a water bath used in Experiment 4?
Provides controlled and uniform heating to prevent unwanted side reactions from overheating
What is Beer’s Law equation in Experiment 4?
A = εbc
Absorbance (A); Concentration (c); Pathlength (b), Molar absorptivity (ε):
What is a calibration curve and why is it important in Experiment 4
A plot of concentration (x) vs absorbance (y) for known standards, to determine the concentration of unknowns
How do you judge if a calibration curve is good or bad in Experiment 4?
A good calibration curve should be a straight line with small y-intercept, transmittance reading of 10% to 90%, and the unknown’s absorbance must fall within the range of the standard solutions
Why is 2 drops of sulfuric acid (H₂SO₄) added to the starting Fe(NH₄)₂(SO₄)₂·6H₂O (iron(II) ammonium sulfate) in Experiment 4?
To prevent premature oxidation
What happens after Oxalic acid (H₂C₂O₄) is added to Fe(II) ammonium sulfate in Experiment 4?
Yellow precipitate of Fe(C₂O₄)·2H₂O forms
Why is heating necessary during synthesis in Experiment 4?
speeds up the reaction and ensures formation of Fe(II)-oxalate before oxidation
What is spectrophotometry? (Experiment 4)
measures how much light a substance absorbs to determine the concentration of the compound
Why is Fe(III)-oxalate difficult to analyze directly using spectrophotometry in Experiment 4
Has a low molar absorptivity (ε) = does not absorb light
How do we make Fe(III) more detectable in spectrophotometry in Experiment 4
Convert Fe(III) to Fe(II)-bipyridine, which forms a deep red complex with strong light absorption.
Why and what happens after you add Bipyridine (bpy) to the secondary stock solution in Experiment 4?
Turns reddish color, allowing Fe(II) to be detectable in the spectrophotometer
Why is ascorbic acid (vitamin C) used the first vs second time in Experiment 4?
Reduces Fe(III) to Fe(II) so it can bind with bipyridine without interfering with the spectroscopic analysis
For control
Why is an acetate buffer used in Experiment 4?
Maintains a stable pH (~4.7) for optimal Fe(II)-bipyridine formation
Why does the pH matter when forming the initial Fe(II)-oxalate in Experiment 4?
pH must be strongly acidic to ensure correct precipitation
Why is Fe(C₂O₄)·2H₂O washed/decanted three times with hot water in Experiment 4?
To remove impurities
Why is hydrogen peroxide (H₂O₂) hazardous when oxidizing Fe(II) to Fe(III)-oxalate in Experiment 4? What to do to prevent danger? (Experiment 4)
Reaction is exothermic and overheating causes decomposition. Therefore, add dropwise, ensuring temperature is at 38-44°C.
How is oxalate removed before spectrophotometric analysis? (Exp 4)
By adding Calcium chloride (CaCl₂) to precipitate calcium oxalate (CaC₂O₄), leaving Fe(III) in the solution.
Why is centrifugation performed in Experiment 4?
Separate the Fe(III) solution from solid calcium oxalate (CaC2O2)
Why must measurements be taken at 520 nm in Experiment 4?
520 nm is the λmax
Why is percent transmittance (%T) recorded instead of absorbance (A) in Experiment 4?
The spectrophotometer has a linear response to %T, which is later converted to absorbance.
Why is the spectrophotometer zeroed with DI water in Experiment 4?
Account for background absorbance for accurate measurements
What happens after you add roughly 3.0 mL of oxalic acid to Fe(III)-Oxalate in Experiment 4?
Forms a yellow-green solution
Why do you need to cover your crystal solution with aluminum foil in Experiment 4?
Crystals are light-sensitive
-Professor’s Example (Experiment 4):
What are the limiting and excess reagents during the synthesis of the green product? Select one.
A) Oxalate is the limiting reagent, and iron is the excess reagent.
B) Oxalate is the limiting reagent, and hydrogen peroxide is the excess reagent.
C) Iron is the limiting reagent, and oxalate is the excess reagent.
D) Iron is the limiting reagent, and hydrogen peroxide is the excess reagent.
E) Iron and oxalate are both the limiting reagents, and hydrogen peroxide is the excess reagent.
Answer: C) Iron is the limiting reagent, and oxalate is the excess reagent
OR
D) Iron is the limiting reagent, and hydrogen peroxide is the excess reagent.
-Professor’s Example (Experiment 4):
During recrystallization, you followed three steps: (1) added ethanol to the reaction mixture to create precipitate, (2) dissolved the precipitate by heating, and (3) allowed the solution to sit unperturbed for at least two days. Why did you go through these three steps? Select one.
A) These steps allowed ethanol to react with water to create a new, pure ligand.
B) These steps allowed ethanol to displace oxalate as the ligand so the sample could be pure.
C) These steps allowed iron to achieve the desired oxidation state for pure crystals.
D) These steps allowed impurities to evaporate and allow pure crystals.
E) These steps allowed the green crystals to emerge slowly while keeping the impurities in solution.
Answer: E) These steps allowed the green crystals to emerge slowly while keeping the impurities in solution.
-Professor’s Example (Experiment 4):
What would be the outcome if you accidentally added water instead of the pH 4.7 buffer to the solution of your green crystal such that some of the iron failed to bind to bipyridine (bpy)? Select one.
A) The mass% of iron would be artificially high.
B) The mass% of iron would be artificially low.
C) The mass% of iron would be artificially high or low, but you cannot predict the direction.
D) The mass% of iron would be unaffected as long as all the iron in the known solutions (for the calibration curve) bound completely to bipyridine.
E) The mass% of iron would be unaffected because the binding of bpy is unrelated to mass% iron.
Answer: Answer: B) The mass% of iron would be artificially low.
Reason: Some Fe(III) may remain unreacted instead of being reduced to Fe(II)
-Professor’s Example (Experiment 4):
Select the two false statements about the calibration data and curve.
A) The calibration data should be fit to an equation for a line.
B) The calibration curve should have a y-intercept close to zero.
C) The calibration curve can be extrapolated to determine the concentration of iron in the green crystals outside the calibration range.
D) The calibration data should be plotted with %T values as the y-axis.
E) The calibration data should be plotted with concentration of iron as the x-axis.
Answer:
C) The calibration curve can be extrapolated to determine the concentration of iron in the green crystals outside the calibration range.
D) The calibration data should be plotted with %T values as the y-axis
Sig Fig Rule for Log/Antilog (Midterm #1)
The number of sig fig in log is the number of decimal place in answer
The number of decimal in antilog is the number of sig fig in answer
Sig Fig Rules for Addition/Subtraction and Multiplication/Division (Midterm #1)
For Addition/Subtraction: Least decimal place
For Multiplication/Division: Least SF
Significiant figures (Midterm #1)
All the certain digits plus the first uncertain digit
Midterm #1:
You have a balance that is incorrectly calibrated and reads weights that are too high. You use this balance to measure the mass of a beaker multiple times. Select one
A) The miscalibrated sample standard can not be calculated
B) You can not predict if the miscalibrated sample standard deviation is smaller or larger than if it was calibrated correctly
C) The misclibrated standard deviation is larger than if it were calibrated correctly
D) The misclibrated standard deviation is same than if it were calibrated correctly
E) The misclibrated standard deviation is smaller than if it were calibrated correctly
Answer: D b/c standard deviation technically doesn’t change
Sig Fig Rule for non-digital instruments w/ Example (Midterm #1)
Exact increment + one more guessing
Answer: 12:3130
Percent Error formula (Midterm #1)
Sample vs population standard deviation (Midterm #1)
Percent Error Definition (Midterm #1)
Provides infomation on the relative accuracy of one measurement to another measurement
-Systematic errors contribute to % error
-Smaller % error = closer to accepted value
-Larger % error = further from accepted value
Note 
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