CHEMISTRY 31 FINAL EXAM
Dec. 10, 2012
150 points total
NAME __________________
Lab Section Number ___________
Constants:
Kw (autoprotolysis of H2O) = 1.0 x 10-14
h = Planck's constant = 6.63 x 10-34 J·s
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c = speed of light in a vacuum = 3.00 x 10 m/s
A. Multiple Choice and Short answer Section. Circle the letter corresponding to the best
answer (only one) or fill in the blank (4 points for each question).
1. A student calculates their unknown soda ash % NaCO3 as 38.331 + 0.091%. When
expressed with the proper number of significant figures, this should be:
a) 40 + 0.09%
b) 38.3 + 0.1
c) 38.33 + 0.09
d) 38.331 + 0.091%
2. It is difficult to prepare sodium hydroxide solutions of known concentration by weighing
solids (because solid NaOH absorbs water so quickly). What can be done to determine the
concentration of NaOH for use in accurate titrations? __________________________________
_____________________________________________________________________________
3. A sample is known to have a matrix effect (a different slope results if using the sample matrix
instead of water to make a calibration curve). One should use a _________ calibration method.
a) standard addition
b) external standard
c) internal standard
d) one point calibration curve
4. Write one complex ion forming reaction used in a quantitative analysis lab.
Reaction: _____________________________________________________________________
5. When ultra-violet light interacts with matter, the predominant type of transition is:
a) nuclear level transitions
b) electron level transitions
c) molecular vibrational transitions
d) molecular rotation transitions
6. In gas chromatography, the two factors affecting a compound's retention factor are:
a) the compound's volatility and polarity
b) the compound's polarity and the mobile phase's polarity
c) the compound's volatility and the flow rate
d) the carrier gas used and its flow rate
7. Which of the following mixtures will make a traditional buffer solution:
a) 0.010 M HCl + 0.010 M NaOH
b) 0.010 M HCl + 0.0050 M NH4Cl
c) 0.030 M HCl + 0.008 M NaCH3CO2
d) 0.0040 M HCl + 0.0090 M NH3
+
(NH4 and CH3CO2H are weak acids)
8. A chemist is measuring the concentration of benzoic acid (a weak acid) in sodas using UV
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absorption. Both benzoic acid standards and soda sample are diluted in a buffer. The main
purpose of the buffer is to:
a) keep the fraction of benzoic acid in the ionized form constant
b) provide additional ions to make ion-pair species that absorb light
c) to increase the ionic strength of the solution
d) to keep the acid from eating through the cuvette walls
9. The compound 2-aminophenol, which exists as NH3C6H4OH+ in its most acidic form, has
pKa1 and pKa2 values of 4.70 and 9.97 for the functional groups -NH3+ and -OH, respectively.
What would be the best pH to use as a buffer so that 2-aminophenol can be retained using anion
chromatography (where it is retained as an anion)?
a) 2.0
b) 5.0
c) 7.0
d) 11.0
10. When a weak base is titrated with a strong acid, the pH at the equivalence point will be
a) below 7
b) exactly 7
c) greater than 7
11. Based on the titration curve shown to the right
(with grey box showing where indicator changes
color), the titration can be considered to be:
a) precise and accurate
b) precise but not accurate
c) accurate but not precise
d) neither accurate nor precise
pH
V(strong base)
Problem Section. Show all needed calculations to receive full credit. The number of points are
shown in parentheses. Use the back side of the page if needed.
1. Calculate the concentration of a 6.0 M formaldehyde (H2CO) solution in mass percent.
Atomic weights (H, C, and O are 1.01, 12.01, and 16.00 g/mol, respectively). The density of the
6.0 M solution is 1.06 g/mL. (8 pts)
2. Calcium fluoride is a sparingly soluble salt that dissociates in water as:
CaF2 (s) ↔ Ca2+ + 2F-. The Ksp for this reaction is 3.9 x 10-11. For both parts of this question
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assume F- and Ca2+ do NOT significantly react with water or form an ion pair.
a) Calculate the solubility of calcium fluoride in water without considering the effects of ionic
strength of the dissolved ions. (8 pts)
b) Determine the solubility of calcium fluoride in 0.050 M KNO3 if the activity coefficients,
γ(Ca2+) and γ(F-), are 0.485 and 0.81, respectively, in the KNO3 solution. (6 pts)
3. A compound is known to have a molar absorbtivity of 731 + 1 M-1 cm-1 at a wavelength of
382 nm in water (solvent). A cell with path length of 0.200 + 0.005 cm is filled with the
compound and the absorbance is measured to be 0.103 + 0.004. Determine the concentration of
the compound and the absolute uncertainty in the concentration and report the numbers with the
correct number of significant figures. (16 pts)
4. Using the HPLC chromatograms for separating components A and B below and the fact that
the separation occurring on column 1 (lower chromatogram) was a reversed phase separation,
answer the questions below. Also, the flow rate for both columns was 1.5 mL/min. and the small
A
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B
first peak seen is for unretained compounds. (12 pts)
3.5
3
response
2.5
2
column 1
column 2
1.5
B
1
0.5
0
0.0
0.5
1.0
1.5
2.0
2.5
time (min.)
Determine:
a) Which compound is more polar: A or B? Explain your answer.
b) The retention factor (k) for A for the separation on column 1.
c) Which column has better resolution? Explain your answer.
Bonus 1)Is the separation occurring on column 2 most likely reversed phase or normal phase?
You can assume that the main factor affecting separation for both columns is polarity. Explain
your answer. (3 pts)
5. A solution of 0.050 M NaBrO is made by dissolving NaBrO into 1.00 L of solution. The Ka
of HBrO (the conjugate acid of BrO-) = 2.3 x 10-9 and Kw = 1.0 x 10-14. Do not consider activity
or any metal complexes to solve this problem to 4 significant figures.
a) Determine the pH of the solution. (10 pts)
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b) If 25 mL of 0.820 M HNO3 is added to the above solution, what will be the pH? (12 pts)
6. Glycine, NH3+CH2CO2-, is an amino acid with an acid (-CO2H) and base (–NH2) functional
group. If the pKas for the –CO2H functional group and –NH3+ (conjugate acid of the base
functional group) are 2.35 and 9.78, respectively, calculate the pH and concentration of
NH3+CH2CO2H (acid form) when 0.010 moles of glycine are used to make a 1.00 L solution.
(12 pts)
Bonus #2) 10 mL of the above solution were added to 90 mL of a buffer. The concentration of the base
form of glycine, NH2CH2CO2-, was measured and found to be 1.2 x 10-4 M. What is the pH of the
buffer assuming the addition of glycine did not affect the pH of the buffer? (3 pts)
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7. A 50.0 mL aliquot of an unknown solution containing the weak acid, formic acid (HCO2H,
with Ka =1.80 x 10-4) is titrated with 0.100 M NaOH and found to require 33.3 mL to reach the
equivalence point. Calculate: (22 pts)
a) The concentration of formic acid in the unknown solution.
b)
The pH at the equivalence point.
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