Question

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

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

Answer 1

Answer: (A)

200 mm

Given molar ratio$=2:5$ v.p $=250mm\left(\text{one solute}\right)$

Moral ratio $=3:4v.p=?\left(\text{another solute}\right)$

Mole fraction of solven $=\frac{5}{2+5}=\frac{5}{4}=0.71\left(1^{st}\text{solute}\right)$

According to Raoult's law;

$p$ (solution)$\times$ mole fraction of solven$\Rightarrow 250=p\left(\text{pure solvent}\right)0.71$

$p\left(\text{pure solvent}\right)=\frac{250}{0.71}=352.11mm$

mole fraction of solvent $=\frac{4}{3+4}=\frac{4}{7}=0.57$

$p\left(\text{solution}\right)=352.11\times 0.57=200.7\approx 200mm$

Hence, the correct option is $A$