Question

If SR = 100 torr and PR = 350 torr, then which of the following cannot be a mole fraction of A in vapour phase and a mole fraction of A in liquid phase respectively?

• A. $\frac{3}{10}$ and $\frac{2}{9}$
• B. $\frac{7}{9}$ and $\frac{3}{10}$
• C. $\frac{2}{9}$ and $\frac{7}{10}$
• D. $\frac{2}{9}$ and $\frac{3}{10}$

Answer 1

Answer: (A), (B), (C)

The correct options are
A $\frac{3}{10}$ and $\frac{2}{9}$
B $\frac{7}{9}$ and $\frac{3}{10}$
C $\frac{2}{9}$ and $\frac{7}{10}$

SR represents the partial pressure of A $\left(P_A\right)$.
It is equal to 100 torr.

PR represents the partial pressure of B.$\left(P_B\right)$.
It is equal to 350 torr.

Hence, the total pressure is $P_T=100torr+350torr=450torr$.

The mole fraction of A in the vapor phase is $Y_A=\frac{P_A}{P_T}=\frac{100torr}{450torr}=\frac{2}{9}$.

The vapor pressure of pure B is 500 torr. The mole fraction of B in the liquid phase is $X_B=\frac{P_B}{P_B^0}=\frac{350torr}{500torr}=\frac{7}{10}$.

The mole fraction of A in the liquid phase is $X_A=1-\frac{7}{10}=\frac{3}{10}$