Question

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

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

Answer 1

Answer: (C)

$140$

Here we will use the simple total vapour pressure equation:

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

$\Rightarrow 84torr=0.8\times 70torr+0.2\times P_B^o$

$\Rightarrow P_B^o=140torr$ .