What is Raoult's law and when does it apply?

• A. P_solution = x_solvent P°_solvent; applies to ideal solutions with a single solvent and nonvolatile solute.
• B. P_solution = x_solvent P°_solvent × total; (nonsense)
• C. P_solution depends on solute volatility only.
• D. P_solution is independent of temperature.

Answer: (A)

Raoult's law says the partial vapor pressure of a component in an ideal solution equals its mole fraction times the vapor pressure of the pure component: P_i = x_i P_i°. When there is a single volatile component (the solvent) and a nonvolatile solute, the solute does not contribute to the vapor pressure. Therefore the total vapor pressure of the solution is just the solvent’s partial pressure: P_solution = x_solvent P°_solvent. This is why the statement is correct: it reflects the ideal-solution behavior in a simple case where only one component drives vapor pressure.

The other ideas aren’t right here: in an ideal solution with a nonvolatile solute, the pressure isn’t an arbitrary “total” factor and isn’t determined by solute volatility (the solute doesn’t vaporize). Vapor pressure also depends on temperature, so P_solution is not independent of temperature. If the solute were volatile, its own partial pressure would contribute to P_solution, so the simple form shown wouldn’t be the full description.