Name: ______________________
Class: _________________
Date: _________
ID: A
CHEM 220 - Exam #3 - Spring 2011
Multiple Choice (5 points each)
Identify the choice that best completes the statement or answers the question.
1. Based on inductive and resonance effects, which of the following will have the smallest Ka value?
A) HBrO3
B) HClO3
C) HBrO2
D) HClO2
E)
All are the same
2. Which compound is acting as the Lewis acid in the following reaction?
A) H2 SO4
B) SO3
C) H2 O
D) None of these.
3. Which of the following 0.010 M solutions will have the highest pH?
A) Na2 CO3
B) HNO2
C) Mg(OH)2
D) NaHCO3
E)
NaOH
4. Which of the following 0.050 M solutions will have the lowest pH?
A) NaH2 PO4
B) NaOH
C) NH3
D) NH4 Cl
E)
NaCl
5. Which of the following are an acid-base conjugate pair that would be useful for creating a buffer with a pH of
11.00?
A)
B)
C)
D)
E)
HCl / NH3
NH3 / NH4 +
CH3 NH2 / CH3 NH3 +
HCl / NaOH
HNO2 / NO2 -
1
Chemistry 220 • Spring 2011 • Exam #3
Name:
FREE RESPONSE: Please answer the following questions in the spaces provided. No answer requires a
quadratic solution. You MAY NOT use your calculator to solve quadratic, cubic, or higher order equations –
apply appropriate approximations when needed. Equilibrium constants do not require units. Otherwise, you
must show all work and units!
1. (20 points) A buffer is prepared by adding solid sodium acetate, NaCH3CO2 (Mm = 82.0)
to 100.0 mL of a 0.25 M solution of acetic acid.
A) What mass of sodium acetate is required to achieve a buffer with a pH = 5.00?
B) If 3.00 mL of 6.0 M NaOH were added to the buffer, what would be the pH of the resulting solution?
2. (35 points) Consider the titration of 30.0 mL of a 0.20 M solution of the weak base, methylamine, CH3NH2,
with 0.50 M HBr.
A) What will be the pH at the equivalence point?
B) What will be the pH when 6.0 mL of HBr has been added?
C) What will be the pH when 10.0 mL of HBr has been added?
D) What will be the pH when 15.0 mL of HBr has been added?
3. What is the concentration of arsenic acid (H3AsO4) in a solution if a 5.00 mL sample of the solution requires
22.83 mL of 0.350 M NaOH to reach the overall (third) equivalence point?
4. Malonic acid is a weak, diprotic acid with the formula HO2CCH2CO2H. For simplicity, you may
abbreviate its formula H2Mal.
A buffer is prepared by dissolving 0.60 g of malonic acid (Mm = 104.1) in 50.0 mL water and
adding 7.0 mL 1.0 M KOH. What is the pH of the buffer?
5) What will be the concentration of the dihydrogen arsenate ion at the overall equivalence point in question
#3? (Note: This answer is independent of the answer to #3).
CONSTANTS & EQUATIONS:
Henderson! Hasselbalch equation:
& [conjugate base] #
!!
pH = pK a + log$$
% [conjugate acid] "
K overall =
K a,forward
K a,reverse
Kw = Ka • Kb
Kw = 1.00 x 10-14
p(X) = -log(X)
The standard Ka/Kb chart given in class was also provided.
A periodic table was provided.