Question

What mass of non-volatile solute (urea) needs to be dissolved in $150g$ of water in order to decrease the vapour pressure of water by $35\%$ . What will be the molality of solution?

• A. $23.88$
• B. $31.12$
• C. $25.47$
• D. $29.92$

Answer 1

Answer: (D)

Using Raoult’s Law:
$\frac{P^o-P_S}{P_S}=\frac{W_{2}M{w}_1}{M{w}_2\cdot W_1}$
If $P^o=100mm$ then $P_S=100-35=65mm$
$\frac{100-65}{65}=\frac{W_2\times 18}{60\times 150}$
$\Rightarrow W_2=\frac{35\times 60\times 150}{65\times 18}=269.23$
$\therefore$ Molality $=\frac{W_2\times 1000}{M{w}_2\times W_1}$
$=\frac{269.23\times 1000}{60\times 150}$
$=29.92m$