Question

At a given temperature, total vapour pressure in Torr of a mixture volatile components A and B is given by
$P_{Total}=120-75X_B$
Hence, vapour pressure of pure A and B respectively (in Torr) are ?

• A. 120, 75
• B. 120, 195
• C. 120, 45
• D. 75, 45

Answer 1

The correct option is C 120, 45

$P_{Total}=P_A{X}_A+P_B{X}_B$
But $X_A=1-X_B$
$(\because X_A+X_B=1)$

Hence, $P_{Total}=P_A(1-X_B)+P_B{X}_B$

$P_{Total}=P_A-P_A{X}_B+P_B{X}_B$

$P_{Total}=P_A-(P_A-P_B)X_B$

But, $P_{Total}=120-75X_B$

Hence, $P_A=120torr$
$P_A-P_B=75$
$P_B=P_A-75$
$P_B=120-75$
$P_B=45torr$