Question

The vapour pressure of pure liquid solvent is $0.80\, atm$. When a non-volatile substance $B$ is added to the solvent, its vapour pressure drops to $0.6\, atm$. What is the mole fraction of component $B$ in this solution?

• A. 0.75
• B. 0.25
• C. 0.48
• D. 0.3

Answer 1

Answer: (B)

0.25

Answer 2

According to Raoult's law, Relative lowering of vapour pressure $\propto$ mole fraction of solute or
$\frac{p^{o}-p_{s}}{p^{0B}}=\chi_{B}$
or $\frac{0.80-0.60}{0.80}=\chi_{B}$
$\therefore \chi_{B}=\frac{1}{4}=0.25$