Version 411 – Exam 1 – McCord – (53215)
1
This printout should have 25 questions.
Multiplechoice questions may continue on
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beFore answering.
McCord
CH302
This exam is only For
McCord’s CH302 class.
Water Data on BACK oF Bubblesheet
001
10.0 points
A semipermeable membrane can withstand
an osmotic pressure oF 0
.
75 atm. What mo
larity oF aqueous magnesium bromide solution
would reach the limit For this membrane? (as
sume
RT
= 25 L
·
atm
·
mol
−
1
).
1.
0
.
01 M
correct
2.
1 M
3.
0
.
03 mM
4.
0
.
03 M
5.
0
.
01 mM
Explanation:
Use the Formula For osmotic pressure. Re
member that magnesium bromide will disso
ciate into 3 particles, so there will be a Van’t
Ho± Factor oF 3 For this calculation.
002
10.0 points
What is the osmotic pressure oF a solution
that contains 4.56
×
10
−
3
moles oF lactose in
100 mL oF solution at M 25
◦
C?
1.
111.5 torr
2.
71 torr
3.
848 torr
correct
4.
536 torr
5.
165.6 torr
Explanation:
n
= 4
.
56
×
10
−
3
mol
V
= 100 mL
T
= 25
◦
C + 273 = 298 K
M
=
p
456
×
10
−
3
mol
100 mL
Pp
1000 mL
L
P
= 4
.
56
×
10
−
2
mol
L
Osmotic pressure
π
=
M R T
=
(
4
.
56
×
10
−
2
mol
/
L
)
×
p
0
.
082
L
·
atm
K
·
mol
P
(298 K)
×
p
760 torr
1 atm
P
= 844 torr
≈
848 torr
003
10.0 points
The vapor pressure oF a liquid at 25
◦
C is 200
torr. IF the pressure over the liquid is lowered
to 190 torr at 25
◦
C, the liquid will
1.
evaporate.
2.
Freeze.
3.
boil.
correct
Explanation:
Vaporization occurs when the vapor pres
sure oF a liquid is less than the pressure oF
the liquid.
Boiling occurs when the vapor
pressure equals that oF the pressure over the
liquid. In this case the vapor pressure exceeds
the pressure over the liquid, so boiling will
occur.
004
10.0 points
What is the boiling point elevation oF a solu
tion oF Na
2
SO
4
(142.1 g/mol, complete disso
ciation) made by dissolving 10.0 g oF Na
2
SO
4
into 250 g water (
K
b
= 0.512
◦
C/
m
)?
1.
0.288
◦
C
2.
0.108
◦
C
3.
0.144
◦
C
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2
4.
0.576
◦
C
5.
0.363
◦
C
6.
0.432
◦
C
correct
Explanation:
m
Na
2
SO
4
= 10.0 g
m
water
= 250 g
MW
Na
2
SO
4
= 142.1 g/mol
m
=
10
142
.
1
0
.
25
= 0
.
2815
m
m
(ideal) = (3)(0
.
2815) = 0
.
844
m
because 3 ions formed. Thus
Δ
T
= (0
.
512)(0
.
844
m
) = 0
.
432
◦
C
005
10.0 points
Consider two liquids A and B. The vapor
pressure of pure A (molecular weight = 50
g/mol) is 225 torr at 25
◦
C and the vapor
pressure of pure B (molecular weight = 75
g/mol) is 90 torr at the same temperature.
What is the total vapor pressure at 25
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 Spring '09
 McCord
 Chemistry, Vapor pressure

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