Question

In a solution with positive deviation vapour pressure of solution is greater than vapour pressure of pure solvent. Explain.

Answer 1

• Non ideal solution showing positive deviation.
• The vapour pressure of a solution is higher than that predicted by Raoult's Law.
• A-B interactions are weaker than those between A-A or B-B, i.e., in this case the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules.
• ​Characteristics of a non-ideal solution are:

(i) $P_A>P_A^o{X}_{A}and{P}_B>P_B^o{X}_B$

(ii) $\Delta H_{mixing}>0$ i.e., solution becomes cold.

(iii) $\Delta V_{mixing}>0$ i.e., expansion of solution takes place.

• Examples of non-ideal solution are:

(i) Acetone and $C{S}_2$

(ii) Acetone and $C_2{H}_{5}OH$

(iii) Acetone and Benzene