Question

$12g$ of urea is present in 1 litre of solution and $68.4g$ of sucrose is separately dissolved in 1 litre of another sample of solution. The lowering of vapour pressure of first solution is:

• (A) Equal to that of second solution
• (B) Greater than that of second solution
• (C) Less than that of second solution
• (D) Double that of seconds solution

Answer 1

The Correct Option is A

Explanation:
Given:Weight of Urea $=12g$And, weight of sucrose $=68.4g$We know that:Number of moles $=\frac{\text{Weight}}{\text{Molar mass}}$Therefore,Moles of urea $=\frac{12}{60}=0.2$(Molar mass of Urea is $60gm$. )Moles of sucrose $=\frac{68.4}{342}=0.2$(Molar mass of sucrose is $342gm$. )Since, there are equal moles of solute in equal volumes of water, the mole fraction is the same. As per Raoult's law, the relative lowering of pressure will also be the same.Hence, the correct option is (A).