Question

What mass of non-volatile solute (urea) needs to be dissolved in $150\, g$ 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:
$\frac{P^{o}-P_{S}}{P_{S}}=\frac{W_{2} Mw_{1}}{Mw_{2}\cdot W_{1}}$
If $P^{o}=100\,mm$ then $P_{S}=100-35=65\,mm$
$\frac{100-65}{65}=\frac{W_{2}\times18}{60\times150}$
$\Rightarrow W_{2}=\frac{35\times60\times150}{65\times18}=269.23$
$\therefore $ Molality $= \frac{W_{2} \times1000}{Mw_{2}\times W_{1}}$
$=\frac{269.23\times1000}{60\times150}$
$=29.92\,m$