Chem 7LM - Final

UC San Diego , Professor Leigh

What are systemic error (Determinate)

Always in the same direction and can (in theory) identify origin

What are random errors (indeterminate)

Results not always same despite seemingly identical conditions. They tend to shift in both directions, have a gaussian distribution, and statistical methods can be used to analyze the data

What are gross errors

They are blunders/mistakes that often leads to outliers

What error affects accuracy

Systemic errors - cause a uniform bias in a set of measurements where results are consistently too high or low.

What error affects random errors

Random errors - cause results to spread ABOVE and BELOW a central value

What is the normal Gauss distribution rule

68-95-99.7

What were you given, your goal, and what chemistry was involved in experiment 2?

Given an aqueous solution of Al(NO3)3 with unknown concentration. The goal was to determine the concentration of Al3+. Solubility was inolved.

What method was used to accomplish the goal of experiment 2

Convert Al³+ (soluble,clear) to Al(hq)3 - insoluble, yellow

Al(NO3)3 9H2O of unknown concentration Al3+ heated. Ammonium acetate buffer added to raise/control pH. Add 6M HCl to help dissolve precipitate. Precipitate forms, and solution covered overnight. Solid vacuum filtrated and weighed

Why did experiment 2 need a pH around 5

Low pH: soluble, but excess H^+ + hq → Hhq

High pH: Correct protonation → hq, but insoluble

Very high pH: 4OH- + Al3+ -→ [Al(OH)4]-

Neutral pH: Hhq (insoluble)

Why was ammonium acetate buffer added to Al3+ + Hhq reaction?

to adjust pH to ~4 and promote Al(hq)₃ precipitation

Calculations done in experiment 2, specifically for the concentration of Al3+

• (mass of crucible + precipitate) - (mass of crucible) = mass of precipitate

• (mass of precipitate)/(molar mass) → mol of Al(hq)3 = mol of Al³+

• mol of Al³+/volume = concentration of Al³+

What were you given, your goal, and chemistry involved in experiment 3

Given an unknown acid, goal was to determine the identity of the acid HA, and will take advantage of acid-base reaction

What method was used for experiment 3

1.) Standarized 0.100M NaOH with KHP

2.) Titrations to identify the molar mass of the acid

3.) Do another titration with a pH meter, tracking pH to identify the pKa

What are the sig figs after calculating a log

Sig figs of answer = number of decimal places

What calculations were done in experiment 3

• (volume NaOH)(molar conc. of NaOH) = moles of NaOH = moles of unknown

• (mass of unknown)/(moles of unknown) = molar mass

• 1st derivative → greatest rate of change = eq. point

What equation was used in experiment 3

Henderson-Hasselbalch Equation: pKa=pH-log[A-]/[HA]

What was given, the goal, and the chemistry used for experiment 4?

Were given Fe(NH4)2(SO4)2 and mass percent of oxalate. The goal is to synthesize Kw[Fex(C2O4)y] zH2O or also known as the Fe(III)-oxalate green crystals and deterime its empirical formula. The chemistry used was spectroscopy. W

What was the method used for experiment 4

• Water bath

  • Add sulfuric acid and oxalic acid (C2H2O4) to starting material (white) → Fe(C2O4) 2H2O (yellow; Fe(II)-oxalate)

• Oxidize Fe(II)-oxalate to Fe(III)-oxalate

  • Add potasium oxalate salt (K2C2O4) and hydrogen peroxide (H2O2) → Fe3+(OH)3 (brown; rust)

  • Remove hydrogen peroxide by boiling

  • Add oxalic acid → Fe(III) oxalate (green; Kw[Fex(C2O4)y])

• Crystalize + recrystallize Fe(III)-oxalate

  • Add ethanol (decreases solubility) → crystals

  • Dissolve in water + heat → crystals

• Vacuum filtration

• Release oxalate ligands

  • Add calcium(II) → Fe3+ ions

  • Fe(II)-bpy is more intensely colored

• Reduce Fe (III) to Fe(II)

  • Add ascorbic acid → Fe2+ ions

  • Add bpy (w/acetate buffer) → Fe(II)-bpy (red)

  • Primary stock: starting material + water

  • Secondary stock: primary + ascorbic acid + bpy

  • Unknown: crystals + water + CaCl2 + ascorbic acid + bpy

  • Add ascorbic acid to secondary stock to match unknown for spectroscopy (any errors will cancel out)

What calculations were made in experiment 4

• Calibration cuve (Fe2+ cocnentratiojn vs. absorbance)

• % mass of oxalate = given

• % mass of iron = calculated from calibration curve

• % mass of potassium = calculated from charge

• % mass of water = leftover mass

• Molecular weight → % yield (>100% due to contaminants)

How do you go from transmittance to absorbance

Abs = -log(transmittance)

What was the goal of experiment 1

Identify glassware that dispenses a specifc volume most accurately and demonstrate that the effect of number of trials on assessment of measurement precision

What happens when n becomes very large to approach N

The value of average should approach the true value of mu

How was the volume of water measures determined in experiment 1

First, mass of empty glass vials was measure. Water was dispensed, then the glass vials were weighed with the water. The mass of water was calculated by subtracting mass full - mass vial. Mass was converted to volume by dividing value by density at recorded temperature

Formula for sample standard deviation

What is the formula for RSD

(standard deviation / average)*100W

What is the percent error formula

|(experimental value) - (true value)| / true value

What is gravimetric analysis

Technique to determine the amount of substance (analyte) dissolved in solution by converting it into solid that can be isolated and weighed

What is a ligand and bidentate ligans

Molecule that donates electron pairs to a metal ion to bind and form a coordination complex. A bidentate ligand can form two bonds to a metal ion

What occues with the hq ligand in experiment 2

The hq ligans deprotonated and bound Al3+ at two sites, forming Al(hq)3 precipitate I

In experiment 2, why is low pH not ideal for ligand binding

Low pH protonates hq, trying up the nitrogen lone pair so it cannot bind Al3+

In experiment 2, why is high pH not ideal for ligand binding

hq binds well, but Al³⁺ forms side products like Al(OH)₃ and Al(OH)₄ instead of the desired precipitate

In experiment 2, what pH of the Al(hq)3 reaction solution was aimed for, and why

pH ~4: because it allowed hq to bind Al³⁺ while minimizing formation of aluminum hydroxide side products, promoting formation of the desired Al(hq)₃ precipitate.

In experiment 2, what was the visual appearance of the Al(hq)3 precipitate after the addition of HCl (expt 2)?

clear, dark orange color

What was the visual appearance of the Al(hq)3 precipitate after the addition of ammonium acetate buffer (expt 2)?

darker yellow color, opaque, contained little floating yellow particles

What is volumetric analysis

Technique to determine identity or concentration of a substance by measuring the volume of a solution that reacts with it

What is a standard solution

a chemical mixture containing a precisely known and highly accurate concentration of a specific solute

What are the limiting and excess reagents of experiment 2

Al3+ = limiting
Hq- = excess

Expt 2: You start with an empty filter flask, and begin to filter trial 1. Halfway through the filtration process of the first trial, you notice that the filter flask contains liquid and yellow solid. What should you do?

Stop what you are doing. Transfer the contents of the filter flask to a secondary beaker and pass the contents of the secondary beaker through the vacuum filtration apparatus again.

Why was the dessicator prepared with anhydrous CaSo4 in experiment 2

To remove moisture from the dessicator to allow the crucibles to cool without absorbing water vapor, which would lead to artificially high mass measurements

Why were the crucibles heated in oven overnight

removes adsorbed moisture from crucible and filter paper to produce a constant dry mass for accurate weighing

Why were the crucibles cooled in the desicator before weighing

Cooling prevents convection errors during weighing (artificially low mass)

How do you calculate a dilution ratio

(value of added)/(value added + total)

How do you read a tritration curve for pH and pka

pH is at the equivalance point. Cut that volume in half, which results at the pKa

Why are buffers important in precipitation reactions

If the pH is not in the desired range, another complex may form contaminating the sample

What can be gained from a semi-mirco scale experiment

Simple procedures (less materials, less waste, and thus greener and more economical chemistry) and quicker analysis (no exact measruments needed only observations)

Why would you do a recrystallization, experiment 4

Recrystallization is a laboratory technique for purifying solids. The key features of this technique is causing a solid to go into solution, and then gradually allowing the dissolved solid to crystallize
- product is not pure
- heat up solution to dissolve both the product and impurities
- when solution cools, the products crashes out and impurities are left behind
- must use the appropriate solvent: pick a compound that will completely dissolve the impurity/contamination

In experiment 4, why was EtOH used instead of water

It does not dissolve crystals, less polar than water, and decreases the solubility of compounds (so crystals don’t dissolve)

In experiment 4, why can’t spectrophotometry be used directly

If the color intensity is weak, the analysis won't be sensitive enough

What does gravimetric analysis rely on?

Precipitate must be of low solubility, high purity, known composition, easy to separate from mixture

Why were was recryslatilzation left overnight

Leave precipitate undisturbed overnight to allow for fewer, larger, and higher purity crystals

What does it mean to do a quantitative transder

all solid in container transferred to another so none is lost, usually using DI water or another specified liquid and a glass rod to guide the solid

In experiment 3, why do we dry the KHP before use

if wet, the mass will be overestimated --> overestimate moles and concentration of NaOH

When it H2O2 a reduced or oxidized

• In acidic conditions (low pH), gets reduced

• In basic conditions (high pH), gets oxidized

In experiment 4, why must H2O2 be added slowly

if added too quickly, the heat liberated from it will decompose it --> the iron will not be completely oxidized, yield will be off

What steps were taken for the syntehsis in experiment 4

How is transmittance (T) calculated

Break down what everything stands for in Beer-Lambert Law

What is the relationship between absorbance with concentration and path length in a spectroscopy. What does it do to transmitted light

• Absorbance (not % transmittance) is linear with respect to concentration

  

• Absorbance (not % transmittance) is linear with respect to pathlength

What are key points for a calibration curve

• Calibration curve should be linear

• The 7L instrument is most reliable 10% to 90%T (1.00 to 0.05 Abs)

• Your data points should fall within the highest and lowest values of the calibration points, which should be subset of 10% to 90% T

• The calibration curve should have an intercept close to zero

• Check the 100% T and 0% T reading intermittently, at least at the end of the experiment

E

D.) Maintain a constant pH to at least 2

D.) The reported concentration of Al3+ will be artificially low because some of the starting material will not have reacted

A.) Gravimetric analysis is defined as quantitative analysis via mass

C.) In the context of this experiment, gravimetric analysis analysis involves conversion of a soluble analyte to an insoluble products

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

D.) Initial pH of the weak acid (before addition of titrant)

D.) 800mg in 80 mL water

D.) You accidentally record the equivalence point as 8.08mL instead of the actual 8.80mL

E.) These steps allowed the green crystals to emerge slowly while keeping the impurities in solution

B.) The mass % of iron would be artificially low

B

C and D