Question

The vapour pressure of pure water is 23.5 mm Hg. Then, the vapour pressure of an aqueous solution which contains 5 mass percent of urea is:

(Molar mass of urea is 60 )

• A. 23 mm Hg
• B. 18 mm Hg
• C. 31 mm Hg
• D. 35 mm Hg

Answer 1

The correct option is A 23 mm Hg

Mass of urea $=5g$
Mass of water $=100-5=95g$

Number of moles of urea $=\frac{5}{60}=0.083$

Number of moles of water $=\frac{95}{18}=5.278$

$\therefore$ Total number of moles $=5.278+0.083=5.361$

Mole fraction of solvent $=\frac{5.278}{5.361}$

$P_s=$ Mole fraction of solvent $\times P^o=\frac{5.278}{5.361}\times 23.5$
$=$ 23.14 mm Hg $\approx$ 23 mm Hg