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

$\begin{array}{1 1}\text{140 Torr}\\\text{280 Torr}\\\text{160 Torr}\\\text{200 Torr}\end{array}$

$p=x_Ap_A^*+x_Bp_B^*$
84 Torr=(0.8)(70 Torr)+(0.2)$(p_B^*)$
$\Rightarrow p_B^*=140$ Torr