Question

If $P^0$ and $P_S$ are the V.P. of solvent and solution respectively and $N_1$ and $N_2$ are the moles of solute and solvent then :

• A. $(P^0-P_S)/P^0=N_1/(N_1+N_2)$
• B. $(P^0-P_S)/P_S=N_1/N_2$
• C. $(P^0-P_S)/P^0=N_1/N_2$
• D. none of these

Answer 1

Answer: (A), (B)

The correct options are
A $(P^0-P_S)/P_S=N_1/N_2$
B $(P^0-P_S)/P^0=N_1/(N_1+N_2)$

If $P^0$ and $P_S$ are the V.P. of solvent and solution respectively and $N_1$ and $N_2$ are the moles of solute and solvent then $(P^0-P_S)/P^0=N_1/(N_1+N_2)$ and $(P^0-P_S)/P_S=N_1/N_2$ where $(P^0-P_S)/P^0$ is relative lowering of vapour pressure.

Thus according to Rault's law;in a solution containing several nonvolatile solutes, the lowering of vapour pressure depends on the sum of the mole fractions of the various solutes. The vapour pressure depends only on the relative number of solute molecules.