Question

The vapour pressure of pure liquid is $70$ Torr at ${27}^{o}C$. The vapour pressure of a solution of this liquid and another liquid (mole fraction 0.2) is $84$ Torr at ${27}^{o}C$. The vapour pressure of pure liquid $B$ at ${27}^{o}C$ is?

• A. $140$ Torr
• B. $280$ Torr
• C. $160$ Torr
• D. $200$ Torr

Answer 1

The correct option is A $140$ Torr

Vapour Pressure of pure liquid $A=70$ torr ($P_A^o$)

Mole fraction of liquid $B=0.2$ ($x_B$)

Thus mole fraction of liquid $A=x_A=1-x_B$

Total vapour pressure $=84$ torr

$P=x_A{P}_A^o+x_B{P}_B^o$

$84=\left(0.8\right)\times 70+0.2\times P_B^o$

$P_B^o$ $=140$ torr $=$ vapour pressure of the pure liquid $B$

Hence, the correct option is $\text{A}$