Question

Two compounds A and B in when mixed form nearly ideal solutions. At room temperature, $P_A^0=40mmHg$ and $P_B^0=120mmHg$.
A liquid mixture is composed of $1mol$ of A and $3mol$ of B.
If the pressure over the mixture at room temperature is reduced. At what pressure does the first vapour form?

• A. $40mmHg$
• B. $120mmHg$
• C. $200mmHg$
• D. $100mmHg$

Answer 1

The correct option is D $100mmHg$

Initially
$x_A=\frac{1}{1+3}=0.25x_B=1-0.25=0.75$
Given:
$P_A^0=40mmHg{P}_B^0=120mmHg$
The pressure at which first vapour is formed is the bubble point pressure.
At bubble point:
When the first vapour is formed, the composition of A and B in the liquid mixture will be the same.
So, at bubble point
$x_A=0.25$
$x_B=0.75$
$P=x_A{P}_A^0+x_B{P}_B^{0}P=0.25\times 40+0.75\times 120P=100mmHg$