A KCl solution is prepared by dissolving 40.0 g KCl in 250.0 g of water at 25°C. What is the vapor pressure of the solution if the vapor pressure of water at 25°C is 23.76 mm Hg?

Psolution = X(mole fraction) x (p solvent)


mole fraction = solute mole / total mole

40 g kcl = .537 mole kcl

250 g h20 = 13.87 mole h20

.537+ 13.87 = 14.407.

mole fraction = .537/14.407 = .037

pressure solvent = 23.76mmHg

.037(23.76)= .87912 mmHg

This is not the right answer, though. What am I doing wrong?

You are confusing solvent with solute. You are using the mole fraction of solute when you need to use the mole fraction of solvent which is water.

Ok, i got the right answer now. I've always known molality to be solute mole// total mole. What lets me know its solvent or solute?

thank you

Solute is the substance that is being dissolved and the solvent is the medium.

I know. I'm confused. I believe sometimes it is solute mole over total mole and sometimes it is solvent mole over total mole. How do I know wbich