Exam 3 SP_08 - Chem 112 Exam 3 SP 2008 Name Multiple choice...

Chem 112 Exam 3 SP 2008 Name _______________________
Multiple choice. 6 points each. Partial credit may be awarded if some work or comments are shown.
Assume 25 o C, aqueous solutions and K
w
= 1.00 x 10 −14 unless otherwise stated.
1. Hydrofluoric acid, HF, is a weak acid with K
a
= 6.8 x 10
−4
. Calculate the pH of a 0.200 M solution of
HF.
a) 0.70 b) 1.17 c) 1.93 d) 3.17 e) 7.00
2. At a temperature where K
w
is 1.4 x 10
−13
, a solution has a pH of 7.00. What is true of this solution?
(circle all that apply)
a) The solution is basic
b) The solution is neutral
c) The solution is acidic
d) Water is less ionized at this temperature than it is at 25 o C.
e) the acidity/alkalinity of the solution is not defined by this information
3. A 0.00100 M solution of lactic acid is prepared in ultrapure water. From high precision conductivity
measurements, the acid is found to be 30.90 % ionized. What is the K
a
for lactic acid?
a) 3.09 x 10
−4
b) 1.38 x 10
−4
c) 9.55 x 10
−5
d) 9.55 x 10
−7
4. A buffer is to be prepared using acetic acid and sodium acetate to a target pH = 5.10. What ratio of the
conjugate−base to conjugate−acid must be used to achieve this pH? K
a
for acetic acid is 1.75 x 10 −5 .
a) 0.34 b) 0.468 c) 2.19 d) 5.10 e) 9.80
5. At equilibrium in a reaction, which of the following statements must be true?
a) the rate of the forward reaction has reached a maximum
b) no reaction is occurring
c) the rate constants of the forward and reverse reactions must be equal
d) there are more products than reactants
e) there is no net change in the amount of any reactant or product

6. K
b
for a certain weak organic base is 5.72 x 10
-
6 . What is K
a
for its conjugate acid at a temperature
where K
w
= 5.01 x 10
-
13 ?
a) 1.75 x 10
-
9 b) 8.74 x 10
-
8 c) 2.80 x 10
-
8 d) 1.14 x 10
-
7
7. Which of the following represents an acid and its conjugate base (conjugate acid/base pair)?
a) H
2
CO
3
/ CO
3
2
-
b) NH
4
+ / NH
3
c) H
3
O+ / OH
-
d) H
2
AsO
4
-
/ AsO
4
3
-
e) HNO
3
/ HNO
2
8. The following reaction has K
eq
= 4.91 x 10
3
for the temperature at which it is occurring, in a 1.00 L
vessel. From the instantaneous concentrations shown, predict what will be observed over time.
2 Al (s) + 6 OH− (aq) ↔ 2 AlO
3
3− (aq) + 3 H
2
(g)
Instantaneous amounts: 0.050 mol 0.0112 mol 5.49x10 −3 mol 0.0125 mol
a) system is at equilibrium. No change in amount of any participant will be observed.
b) system is NOT at equilibrium. More products will form over time
c) system is NOT at equilibrium. More reactants will form over time.
d) temperature and pressure must be increased to achieve equilibrium
e) the situation with respect to equilibrium cannot be determined from the information given
9. A solution is prepared from aluminum acetate Al(CH
3
COO)
3
. The pK
a
for aluminum ion Al 3+ (aq) is:
pK
a
= 4.95 and for acetic acid: CH
3
COOH pK
a
= 4.76
What is likely true about this solution? (both parts of statement must be true for the statement to be true.)
a)