Chemistry 222
Exam 3: Chapters 8-10
80 Points
Name__________________________________________
Fall 2015
Complete five (5) of the following problems. Each problem is worth 16 points. CLEARLY mark
the problem you do not want graded. You must show your work to receive credit for problems
requiring math. Report your answers with the appropriate number of significant figures.
1. Calculate the pH for each of the following cases in the titration of 25.0 mL of 0.180 M
pyridine, C5H5N(aq) (pKb = 8.80) with 0.180 M HBr(aq): (8 points for each part)
a. At the equivalence point
b. After the addition of 22.0 mL of HBr
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2. For this problem consider the newly discovered kick acid, a triprotic weak acid of the form
H3A.
a. The plot below represents the fraction of dissociation for kick acid. Label the four curves
with their corresponding alphas (H3A, H2A-, HA2-, A3-) and determine the values for
pKa1, pKa2, and pKa3. (6 points)
b. Based in your results from part a, use the axes below to sketch the titration curve for the
titration of 20.0 mL of a 0.100 M solution of kick acid with 0.100 M NaOH. Specify the
pH at a minimum of five points on the plot. (10 points)
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3. Answer the following: (8 points each)
a. Why is a buffer solution most effective at a pH within ±1 of the pKa for the acid used to
make the buffer?
b. Why is the pH of a buffer effectively independent of dilution, while the capacity of the
buffer diminishes as it becomes more dilute?
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4. You need to prepare a pH 4.50 buffer by adjusting the pH of 200.0 mL of a 0.200 M solution
of monosodium oxalate (NaHC2O4). The only acid and base solutions you have available
are 100 mL of 0.100 M NaOH, and 100 mL of 0.100 M HCl. Given these solutions, can you
make your buffer? Justify your answer with appropriate calculations.
(For oxalic acid, pKa1 = 1.25, pKa2 = 4.26.)
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5. Calculate the pH of the following solutions: (8 points each)
a. A 2.2 x 10-7 F solution of HCl.
b. A solution prepared by combining (a) 12.00 mL of 1.00 M NaOH; (b) 20.0 mL of 0.200 M
H2SO4; (c) 5 mL 0.5 M NaNO3; (d) 2.00 mL 1.00 M HCl and (e) 10.00 mL 0.800 M
benzoic acid and diluting to 100.0 mL. The pKa for benzoic acid is 4.20.
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6. Calculate the pH and concentrations (in moles per liter) of H2A, HA- and A2- in a solution
prepared by dissolving 0.200 moles of NaHA in 1.00 L of solution.
(For H2A: Ka1 = 1.0x10-3, Ka2 = 1.0x10-7)
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Possibly Useful Information
H  

K a1K a 2F  K a1K w
 K a1K a 2
K a1  F
pH 
conjugate base 
pH  pK a  log
weak acid 
 A 2 
 H2 A
H 

H   H  K
 2
 2
H   H  K

a1

a1
K a1K a 2
 2
1
pK a1  pK a 2 
2
 K a1K a2
K aK b = K w
 K a1K a 2
Kw = 1.0 x 1014 = [H+][OH-]
x
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 b  b 2  4ac
2a