[aniel]

Calculate the vapor pressure at 30 degrees Celsius of a 3.0 molal solution of ethylene glycol assuming that the vapor pressure of a solution containing a non-volatile solute is proportional to the mole fraction of the solvent.

[sjb]

Calculate the vapor pressure at 30 degrees Celsius of a 3.0 molal solution of ethylene glycol assuming that the vapor pressure of a solution containing a non-volatile solute is proportional to the mole fraction of the solvent.

Have you checked the forum rules? Any equations you know that may help?

[aniel]

for m or molality , the formula is m=wt. of solute/ MW(kg.solvent)
*MW-> molecular weight

P_a= X_a (P_a°)

where P_a = vapor pressure of the soln
X_a =mole fraction of the solvent
P_a°= vapor pressure of the solvent

[aniel]

the solvent is assumed as water

[Borek]

You have to convert molality to the molar fraction.

Assume solution containing 1 kg of water.

How many moles of glycol?

How many moles of water?

What is molar fraction of water?

[aniel]

is this correct?

there are 0.3 moles of glycol; 55.5556 moles of water

and the mole fraction of water is 0.9946

[aniel]

do we always assume that the water(solvent) is 1kg?

[Borek]

there are 0.3 moles of glycol

Why 0.3?

do we always assume that the water(solvent) is 1kg?

Not necessarily, but that's the most convenient thing to do, as then it is very easy to calculate number of moles of the solute.