CHEM 213
Chemical Analysis
Exam 2
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(please print)
1. Use activites (i.e., consult Kielland’s Table of Activity Coefficients on page 8 of
this exam) to calculate the molar solubility of Zn(OH)2 (Ksp = 3.0•10-16) in
b. the solution that results when you mix 20.0 mL of 0.100 M KOH with 80
mL of 0.0250 M ZnCl2. (5 points)
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2. Consider the following equilibria, in which all ions are aqueous:
(1) Ag+ + Cl- ↔ AgCl(aq)
K = 2.0•103
(2) AgCl(aq) + Cl ↔ AgCl2 K = 9.3•101
(3) AgCl(s) ↔ Ag+ + ClK = 1.8•10-10
a. Calculate the numerical value of the equilibrium constant for the reaction
AgCl(s) ↔ AgCl(aq) (4 points)
b. Calculate the concentration of AgCl(aq) in equilibrium with excess
undissolved solid AgCl. (2 points)
c. Find the numerical value of K for the reaction
AgCl2- ↔ AgCl(s) + Cl-
(4 points)
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3. (a) Find the pH of a solution prepared by dissolving 12.43 g of tris (MW =
121.135 g/mol) plus 4.67 g of tris hydrochloride (MW = 157.596 g/mol, Ka =
8.4•10-9) in 1.00 L of water. (5 points)
(b) If we add 12.0 mL of 1.00 M HCl to the solution made in (a), what will be the
new pH? (5 points)
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4. A solution with an ionic strength of 0.10 M containing 0.010 M
phenylhydrazine has a pH of 8.13. Using activity coefficients correctly, find
the pKa for the phenylhydrazonium ion found in phenylhydrazine
hydrochloride. Assume that γBH+ = 0.80 (appropriate value for OH- can be
found in Kiellands table, page 8). (10 points)
NHNH2
Phenylhydrazine
B
NHNH3+ClPhenylhydrazine hydrochloride
BH+Cl-
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5. Calculate the solubility product of silver chromate (Ag2CrO4, MW 331.7 g/mol),
given that its solubility is 2.5•10-2 g/L. (5 points)
Ag2CrO4(s) ↔ 2Ag+(aq) + CrO42-(aq)
6. Use the Debye-Hückel equation to calculate the activity coefficient of Fe3+ at µ
= 0.075 (αFe3+ = 0.9 nm). (5 points)
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7. Calculate the pH at each point listed for the titration of 50 mL of 0.050 M
formic acid (HCO2H, MW = 46.02, Ka = 1.80•10-4) with 0.050 M KOH. The
points to calculate are Va = 0.0, 10.0, 25.0, 48.0, 50.0, 50.5, and 60.0 mL. (10
points)
The titration reaction is:
HCO2H + OH- → HCO2- + H2O
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8. How many grams of Na2HPO4•2H2O (MW = 177.99 g/mol) must be added to
300 mL of 0.200 M H3PO4 to give a buffer of 7.30? Ka1(H3PO4) = 7.11•10-3,
Ka2(H3PO4) = 6.32•10-8, Ka3(H3PO4) = 4.5•10-13 (10 points)
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9. How would you prepare 1.00 L of a buffer with pH of 7.00 from 0.200 H3PO4
and 0.160 M NaOH? Ka1(H3PO4) = 7.11•10-3, Ka2(H3PO4) = 6.32•10-8,
Ka3(H3PO4) = 4.5•10-13 (10 points)
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