Describe Raoult's law for ideal solutions and what deviations indicate.

• A. P_i = x_i P_i°; deviations indicate non-ideality due to molecular interactions.
• B. P_i = x_i P_i°; deviations indicate ideal behavior.
• C. The law states that the partial pressure of each component is proportional to its mole fraction times its vapor pressure if ideal; deviations indicate non-ideality.
• D. The law only applies to ideal gases in the gas phase.

Answer: (C)

Raoult's law describes how the vapor pressure of each component in an ideal liquid solution relates to its presence in the liquid. In an ideal mixture, the partial pressure of a component is simply its mole fraction in the liquid times its vapor pressure as a pure substance at the same temperature: P_i = x_i P_i°. Because there are no special interactions between different species beyond those in the pure liquids, the total vapor pressure is the sum of these partial pressures: P_total = Σ x_i P_i°.

Deviations from this ideal relation signal non-ideality in the liquid phase. A positive deviation means the components interact with each other less strongly than they do in their pure liquids, so each component tends to escape more readily and its partial pressure is higher than the ideal prediction. A negative deviation indicates stronger interactions between unlike molecules, which suppress volatility and lower the partial pressures below the ideal values. These deviations reveal the presence of specific intermolecular forces or structural effects that aren’t captured by the simple proportionality.