Question

The vapour pressure of a pure liquid $A$ is $70$ torr at ${27}^{0}C$. It forms an ideal solution with another liquid B. The mole fraction of B is $0.2$ and the total vapour pressure of the solution is $84$ torr at ${27}^{0}C$. The vapour pressure of pure liquid $B$ at ${27}^{0}C$ is:

• A. $14$
• B. $56$
• C. $140$
• D. $70$

Answer 1

The correct option is C $140$

$P_A^0=70torr,X_B=0.2,X_A=0.8,P_t=84torr$

$84=X_A{P}_A^0+X_A{P}_B^0$

$84=0.8\times 70+0.2\times x$

$84=56+0.2\times x$

$x=140torr$

Hence $140$ torr is the vapour pressure of pure $B$.