Q: If P_{0} & P_{S} be the vapour pressure of solvent and its solution respectively and N_{1} and N_{2} be the mole-fractions of solvent and solute respectively , then:

(A) P_{S} = P_{0} N_{2}

(B) P_{0} = P_{S} = P . N_{2}

(C) P_{S} = P_{0} N_{1}

(D) $\large \frac{P_0 – P_S}{P_S} = \frac{N_1}{N_1 + N_2} $

Solution: According to Raoult’s Laws

Vapour pressure of solution = V.P. of solvent X solvent

$\large \frac{P_0 – P_S}{P_0} = N_2 $

$\large P_0 – P_S = P_0 \times N_2 $

$\large P_S = P_0 – P_0 \times N_2 $

$\large P_S = P_0 (1 – N_2 ) = P_0 N_1 $ (Since , N_{1} + N_{2} = 1)

Answer is (C)