Question

A liquid s has a vapour pressure p at 30c. If x gm of non-volatile solute is dissolved in y gm of pure liquid, the vapour pressure becomes th of its original value. Assuming the molecular weight of the solvent is th of the molecular weight of solute. The ratio x : y is: a 1 : 20 b 3 : 20 c 4 : 20 d 5 : 20

Answer 1

Answer:b3:20

Explanation:Let us consider a solution obtained by dissolving n moles of a non-volatile solute in N moles of a volatile solvent. Then mole fraction of the solvent, X 1 = N/(n+N) and mole fraction of the solute, X2 = n /(N +n). Since the solute is non-volatile, it would have negligible vapour pressure. The vapour pressure of the solution is, therefore merely the vapour pressure of the solvent. According to Raoult's law, the vapour pressure of a solvent (P1) in an ideal solution is given by the expression ;

P1 = X1 P10 ................(1)

where P10 is the vapour pressure of the pure solvent. Since X1 + X 2 = 1, Eq. 1 may be written as