Make up Exam 1-solutions - Version 042 Make up Exam 1...

Version 042 – Make up Exam 1 – Laude – (52390) 1
This print-out should have 28 questions.
Multiple-choice questions may continue on
the next column or page – find all choices
before answering.
001 6.0 points
Rank the following liquids in increasing
order of solubility in water (H
2
O): CH
3
F,
(CH
3
)
2
NH, CH
3
Cl, CBr
4
.
1. (CH
3
)
2
NH < CBr
4
< CH
3
Cl < CH
3
F
2. CBr
4
< CH
3
Cl < CH
3
F < (CH
3
)
2
NH
correct
3. CH
3
Cl < CH
3
F < (CH
3
)
2
NH < CBr
4
4. CH
3
F < (CH
3
)
2
NH < CBr
4
< CH
3
Cl
Explanation:
According the axiom that ”like dissolves
like,” the least miscible will be the non-polar
molecule CBr
4
. The most miscible will be
(CH
3
)
2
NH, which exhibits hydrogen bonding
just as water does. Between the two polar
molecules, CH
3
F is the more polar and thus
the more miscible because of the greater elec-
tronegativity difference.
002 6.0 points
Which of the following statements is/are
always true concerning K
w
?
I) It gets larger as the temperature in-
creases
II) It equals 10
− 14
III) K
w
= [H
+
][OH
−
]
1. I, II, III
2. II, III
3. I only
4. II only
5. I, III correct
6. III only
7. I, II
Explanation:
All equilibrium processes are temperature
dependent, and because auto-protolysis is en-
dothermic, K
w
increases as temperature in-
crease. Thus statement I is true, but state-
ment II is only true at room temperature.
Statement III is the definition of K
w
.
003 6.0 points
An unknown liquid has a vapor pressure of
88 mmHg at 45
◦
C and a heat of vaporization
of 32 kJ / mol, what is its vapor pressure at
55
◦
C?
1. 156 mmHg
2. 127 mmHg correct
3. 61 mmHg
4. 89 . 4 mmHg
Explanation:
Use the Clausius-Clapeyron Equation.
Here, the only thing we don’t know is P
2
.
ln
parenleftbigg
P
2
P
1
parenrightbigg
=
Δ H
vap
R
parenleftbigg
1
T
1
−
1
T
2
parenrightbigg
ln
parenleftbigg
P
2
88
parenrightbigg
=
32 , 000
8 . 31
parenleftbigg
1
318
−
1
328
parenrightbigg
ln P
2
=
32 , 000
8 . 31
parenleftbigg
1
318
−
1
328
parenrightbigg
+ ln 88
P
2
= e
4 . 84
= 127 mmHg
004 6.0 points
What would be the pH of a solution of hy-
poiodous acid (HOI) prepared by dissolving
144 grams of the acid in 200 mL of pure
water (H
2
O)? The Ka of hypoiodous acid is
2 × 10
− 11
1. 7
2. 13
3. 1

Version 042 – Make up Exam 1 – Laude – (52390) 2
4. 10
5. 5 correct
Explanation:
144 g HOI ×
1 mol
144 g
= 1 mol HOI
1 mol HOI
0 . 2 L H
2
O
= 5 M HOI
[H
+
] = (K
a
· C
a
)
1 / 2
= (2 × 10
− 11
· 5)
1 / 2
= (10
− 10
)
1 / 2
= 10
− 5
pH = − log[H
+
] = − log(10
− 5
) = 5
005 6.0 points
Which of the following would lower the va-
por pressure of a sample of water in a closed
container?
I) decreasing the size of the container
II) lowering the container temperature
III) removing water from the container
1. I only
2. II and III only
3. I and III only
4. III only
5. I and II only
6. I, II and III
7. II onlt correct
Explanation:
The size of a container and the amount of
the volatile species contained in it are irrel-
evant to vapor pressure. Temperature is a
strong determinant vapor pressure.