Thompson, Margaret – Homework 2 – Due: Sep 18 2007, 11:00 am – Inst: James Holcombe
1
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printout
should
have
25
questions.
Multiplechoice questions may continue on
the next column or page – find all choices
before answering.
The due time is Central
time.
001
(part 1 of 1) 10 points
The temperature at which water boils can be
increased by adding some solute, like ethylene
glycol. This is known as a colligative property.
1.
False
2.
True
correct
Explanation:
Colligative properties depend on the num
ber of particles dissolved in the solution, not
on the properties of the components.
This colligative property is the boiling point
elevation of water.
002
(part 1 of 1) 10 points
The colligative effects of 1 molal sugar solu
tion would be
?
1 molal sodium chloride
solution.
1.
less than
correct
2.
the same as
3.
greater than
Explanation:
Sugar is a nonelectrolyte so the concen
tration of particles is the concentration of
dissolved sugar.
Soduim chloride is an electrolyte which dis
sociates in solution, giving twice as many par
ticles as dissolved:
NaCl
→
Na
+
+ Cl

Since colligative properties depend on the
number of particles in solution, the colligative
effect doubles for the NaCl(aq), compared to
any identical concentration of sugar.
003
(part 1 of 1) 10 points
The vapor pressure of a volatile component
in a solution decreases as its mole fraction
decreases. This is known as
1.
LeChatelier’s Principle.
2.
Bragg’s Law.
3.
Raoult’s Law.
correct
4.
Henry’s Law.
Explanation:
According to Raoult’s Law, the vapor pres
sure of the volatile component of a solution
equals its mole fraction in the solution times
the vapor pressure of the pure solvent.
P
=
X P
0
004
(part 1 of 3) 10 points
Consider the following pressure diagram for a
binary liquid containing A and B.
0
1
x
A
90 torr
30 torr
If 800 g of A (100 g
/
mol) and 1140 g of
B (95 g
/
mol) are mixed, what is the vapor
pressure of B?
1.
12 torr
2.
90 torr
3.
65 torr
4.
53 torr
correct
5.
30 torr
Explanation:
We
have
800 g
100 g
/
mol
=
8
mol
A
and
1140 g
95 g
/
mol
= 12 mol B, so the mole fraction
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Thompson, Margaret – Homework 2 – Due: Sep 18 2007, 11:00 am – Inst: James Holcombe
2
of A is
x
A
=
mol A
total mol
=
8 mol
8 mol + 12 mol
= 0
.
4
0
1
0
.
4
30 torr
53 torr
005
(part 2 of 3) 10 points
What is the corresponding vapor pressure of
A?
1.
90 torr
2.
30 torr
3.
65 torr
4.
12 torr
correct
5.
53 torr
Explanation:
0
1
0
.
4
30 torr
12 torr
006
(part 3 of 3) 10 points
What is the corresponding total vapor pres
sure?
1.
53 torr
2.
12 torr
3.
30 torr
4.
65 torr
correct
5.
90 torr
Explanation:
0
1
0
.
4
30 torr
65 torr
007
(part 1 of 1) 10 points
1,1Dichloroethane (CH
3
CHCl
2
) has a va
por pressure of 228 Torr at 25
◦
C; at the same
temperature,
1,1dichlorotetrafluoroethane
(CF
3
CCl
2
F) has a vapor pressure of 79 Torr.
What mass of 1,1dichloroethane
must be
mixed with 280 g of 1,1dichlorotetrafluoro
ethane to give give a solution with vapor
pressure 157 Torr at 25
◦
C?
Assume ideal
behavior.
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 Spring '07
 Holcombe
 Chemistry, Vapor pressure, Freezingpoint depression

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