The vapor pressure of pure water at 30°C is 40 torr. If NaCl is dissolved into a sample of water until the total vapor pressure of the mixture at 30°C is 32 torr, what is the mole fraction of NaCl?

Mole fraction is 0.2

Explanation:

You have to apply the colligative property about vapour pressure which formula is:

ΔP = P°. Xm

Where ΔP = P° - P

Pressure of vapour from pure solvent (P°) - Pressure of vapour from solution

40 Torr - 32 Torr = 40 Torr. Xm

8 Torr / 40 Torr = 0.2. > Xm