If the vapor pressure of the pure water was .73atm, what is the vapor pressure after the addition of the sodium chloride?
A. .342
B. .688
C. .73
D. .942
B
In general, the addition of a solute lowers the vapor pressure, immediately eliminating choices C and D. The new vapor pressure is given by Raoult's law. Since the vapor pressure of the solvent is needed, the mole fraction of the solvent is used.