Question

Assuming very dilute aqueous solution of urea, calculate the vapour pressure of solution (in mm of Hg) of 0.1 moles of urea in $180 \, grams$ of water at $25^{\text{o}}\text{C}$ is (The vapour pressure of water at $25^{\text{o}}\text{C}$ is 24 mm Hg)

• A. 2.376
• B. 20.76
• C. 23.76
• D. 24.76

Answer 1

Answer: (C)

From Raoult's law for very dilute solution.

$\frac{\text{p}^{\text{o}} - \text{p}_{\text{s}}}{\text{p}^{\text{o}}}=\frac{\text{w}}{\text{m}}\times \frac{\text{M}}{\text{W}}$

$\frac{24 - \text{p}_{\text{s}}}{24}=0.1\times \frac{18}{180}$

$24-\text{p}_{\text{s}}=0.24$

$\therefore $ $\text{p}_{\text{s}}=24-0.24=23.76 \, \text{m}\text{m} \, \text{H}\text{g}$