Question

A solution of two miscible liquids showing negative deviation from Raoult's law will have:

• A. Increased vapour pressure, increased boiling point
• B. Increased vapour pressure, decreased boiling point
• C. Decreased vapour pressure, decreased boiling point
• D. Decreased vapour pressure, increased boiling point

Answer 1

Answer: (D)

Decreased vapour pressure, increased boiling point

Answer 2

A solution showing negative deviation from Raoult’s law has lower vapour pressure than expected because the interactions between the components are stronger than in the pure liquids. This results in a decreased vapour pressure, which leads to an increase in boiling point.

For solutions showing negative deviation:

$P_T < P_A^0 X_A + P_B^0 X_B$

where $P_T$ is the total vapour pressure, $P_A^0$ and $P_B^0$ are the vapour pressures of pure components, and $X_A$ and $X_B$ are their mole fractions.