Liquids A and B form an ideal solution. A certain solution of A and B contains 25 mole per cent of A whereas the vapours in the equilibrium at 298 K contains 50 mole per cent of A. The ratio of vapour

Question

Liquids A and B form an ideal solution. A certain solution of A and B contains 25 mole per cent of A whereas the vapours in the equilibrium at 298 K contains 50 mole per cent of A. The ratio of vapour pressures of pure A to that of pure B at 298 K, is

Prashant Kotian · Accepted Answer

Correct answer: B. Mole fraction of A in liquid phase X_A = 0.25 => X_B = 0.75. Mole fraction of A in vapour phase Y_A = 0.50 => Y_B = 0.50. According to Raoult's and Dalton's laws: Y_A / Y_B = (P_A^o * X_A) / (P_B^o * X_B) => 0.50 / 0.50 = (P_A^o * 0.25) / (P_B^o * 0.75) => 1 = (P_A^o / P_B^o) * (1 / 3) => P_A^o / P_B^o = 3. Thus, the ratio of vapour pressures is 3:1.

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