5. The vapor pressure of an aqueous solution is found to be 24.9 mmHg at 25°C. W
ID: 1009290 • Letter: 5
Question
5. The vapor pressure of an aqueous solution is found to be 24.9 mmHg at 25°C. What is the mole fraction of solute in this solution? The vapor pressure of water is 25.756 mmHg at 25°C.
a.
0.967
d.
0.034
b.
0.033
e.
0.976
c.
1.03
____ 6. The normal freezing point of benzene (C6H6, 78.12 g/mol) is 5.49°C, and its Kf value is 5.12°C/m. To create a solution with a freezing point of 0.00°C, how many grams of naphthalene (C10H8, 128.2 g/mol) would need to be dissolved in 640.0 g benzene?
a.
5.63 g
d.
187 g
b.
137 g
e.
88.0 g
c.
82.0 g
____ 7. Ethylene glycol is used in automobile radiators as an antifreeze. Use the following information to determine the freezing point of an antifreeze solution made by mixing 2.5 L of ethylene glycol with 2.5 L of water. Ethylene glycol is essentially nonvolatile and it does not dissociate in water.
molar mass of ethylene glycol = 62.1 g/mol
density of ethylene glycol = 1.11 g/mL
density of water = 1.00 g/mL
Kf for water = 1.86°C kg/mol
a.
–39°C
d.
–33°C
b.
–16°C
e.
–24°C
c.
–1.6°C
____ 8. Which statement regarding osmotic pressure is NOT correct? Osmotic pressure __________
a.
increases with increasing temperature.
b.
increases with increasing molar concentration of solute.
c.
is greater for 0.1 M Na2SO4 than for 0.1 M NaCl.
d.
depends on the value of the gas constant R.
e.
is the same for solutions with the same mass percent of solute.
____ 9. Identify the following statement as true or false and choose the correct explanation: “For solutions with the same molarity at the same temperature, the pressure needed for reverse osmosis of a sodium chloride solution will always be less than the pressure needed for a calcium chloride solution.”
a.
True, because the molar mass of sodium chloride is smaller.
b.
True, because the van ’t Hoff factor is smaller for sodium chloride.
c.
False, because the van ’t Hoff factor is smaller for sodium chloride.
d.
False, because the van ’t Hoff factor is larger for sodium chloride.
e.
True, because the van ’t Hoff factor is larger for sodium chloride.
____ 10. A 376 mg sample of a nonelectrolyte compound isolated from throat lozenges was dissolved in water to produce 10.0 mL of a solution at 25°C. The osmotic pressure of this solution was measured and found to be 4.89 atm. What is the molar mass of this compound?
a.
188 g/mol
d.
199 g/mol
b.
941 g/mol
e.
489 g/mol
c.
48.9 g/mol
a.
0.967
d.
0.034
b.
0.033
e.
0.976
c.
1.03
Explanation / Answer
5. P0-Ps/P0 = X
(25.756-24.9)/25.756 = x
x = 0.033
answer: b
6. DTf = i*kf*m
(5.49-0) = 1*5.12*(w/128.2)*(1000/640)
w = mass of naphthalene = 87.98 grams <------ answer
7.
mass of glycol = 2500*1.11 = 2775 grams
mass of water = 2500*1 = 2500 grams
DTf = i*kf*m
(0-x) = 1*1.86*(2775/62.1)*(1000/2500)
x = Freezing point of solution = -33.25 C <------ answer
8.
e. is the same for solutions with the same mass percent of solute.
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