Question

In ideal solution of non volatile solute $B$ in solvent $A$ in $2:5$ molar ratio has vapour pressure $250mm$ . If an another solution in ratio $3:4$ prepared then vapour pressure above this solution

• A. 200 mm
• B. 250 mm
• C. 350 mm
• D. 400 mm

Answer 1

Answer: (A)

$\frac{n_B}{n_A}=\frac{2}{5},$
$P_s=250,$
$\frac{n_B}{n_A}=\frac{3}{4}$
$P_s=?$
$P_s=p_A+\left(p_e\right)⟶$ non-volatile solute $p_e=0$
$P_s=p_A+p_x^{\circ }\times x_A$
$x_A=\frac{n_A}{n_A+n_B}=\frac{5}{5+2}=\frac{5}{7}...(i)$
$x_A=\frac{n_A}{n_A+n_B}=\frac{4}{4+3}=\frac{4}{7}...(ii)$
$250=P_s=p_A^{\circ }\times \frac{5}{7}...(iii)$
$P_s=p_A^{\circ }\times \frac{4}{7}...(iv)$
$\frac{1}{2}\to \frac{250=p_A^{\circ }\times \frac{5}{7}}{P_s=p_A^{\circ }\times \frac{4}{7}}$
$\frac{250}{P_s}=\frac{5}{4}$
$P_s=\frac{250\times 4}{5}=200mm$