[demonat0r]

Which of the following will have the lowest total vapor pressure at 25 degree Celcius?
a) pure water (Vapor pressure = 23.8 torr at 25 degree C)
b) a solution of glucose in water with mole fraction of glucose = 0.01
c) a solution of sodium chloride in water with the mole fraction of NaCl = 0.01
d) a solution of methanol in water with mole fraction of methanol = 0.2 (Consider the vapor pressure of both methanol (143 torr at 25 degree C) and water)

i calculated the vapor pressure of each and they turned out to be:

a) 23.8 torr
b) (.99) (23.8 torr) = 23.562 torr
c) (.99) (23.8 torr) = 23.562 torr
d) (0.2)(143) + (0.8 )(23.8 ) = 47.64 torr

and it seems like both b) and c) the solutions of glucose and sodium chloride respectively have the lowest total vapor pressure. but my textbook says that the answer to this problem is just c). if possible please explain why the answer is only c) and not b) and c). did i miss something or make a wrong calculation? thanks

[demonat0r]

please help me with this problem.

[sjb]

What equations did you use to establish the maths in your answers?

[demonat0r]

What equations did you use to establish the maths in your answers?

i used Raoult's Law of Vapor Pressure.
for a nonvolatile solute, it's:
Pressure of solution = (mole fraction of solvent) (partial pressure of pure solvent)

for liquid-liquid solutions where both components are volatile, it's:
Pressure of solution = [(partial pressure of solute)(mole fraction of solute)] + [(partial pressure of solvent)(mole fraction of solvent)]

[sjb]

Considering simply b and c, what is the composition of the solutions you have?

[demonat0r]

Considering simply b and c, what is the composition of the solutions you have?

b) solution of glucose in water and the mole fraction of glucose is given as 0.01. therefore i said that the mole fraction of water in the solution should be 0.99.

c) solution of NaCl in water and the mole fraction of NaCl is given as 0.01. therefore i said that the mole fraction of water in the solution should be 0.99.

since they have the same mole fraction of water, they should have the same vapor pressure, but the answer in the back of my book says otherwise.

[sjb]

What happens when glucose dissolves in water?

What happens when salt dissolves in water?

[demonat0r]

What happens when glucose dissolves in water?

What happens when salt dissolves in water?

well glucose dissolves but NaCl dissociates into Na+ and Cl- ions.

[sjb]

What happens when glucose dissolves in water?

What happens when salt dissolves in water?

well glucose dissolves but NaCl dissociates into Na+ and Cl- ions.

So if the mole fraction of NaCl is 0.01, what is the mole fraction of Na⁺, and of Cl^-?

[demonat0r]

if the mole fraction of NaCl is 0.01, the mole fraction of Na+ would be 0.005 and the mole fraction of Cl- would be 0.005.

[demonat0r]

i still don't see why c) the solution of NaCl in water has the lowest total vapor pressure because since both b) (solution of glucose in water) and c) have the same mole fraction of solute and thus the same mole fraction of solvent, and using Raoult's Law: Pressure solution = (mole fraction of solvent)(partial pressure of solvent), they turn out to have the same vapor pressure. i don't understand why c) is the correct answer according to my textbook instead of b) and c).

[Borek]

if the mole fraction of NaCl is 0.01, the mole fraction of Na+ would be 0.005 and the mole fraction of Cl- would be 0.005.

No.

Let's say you had 0.99 mole of water and 0.01 mole of NaCl. You mix them and you get solution that has mole fraction of NaCl 0.01.

But now NaCl dissosiate. You have 0.99 mole of water, 0.01 mole of Na⁺ and 0.01 mole of Cl^-. What are mole fractions of Na⁺ and Cl^-?