Question

At $100^{\circ} C$ the vapour pressure of a solution of $6.5 \,g$ of a solute in $100 \,g$ water is $732\, mm$. If $K_b = 0.52$, the boiling point of this solution will be:

• A. $100^{\circ} C$
• B. $102^{\circ} C$
• C. $103^{\circ} C$
• D. $101^{\circ} C$

Answer 1

Answer: (D)

$101^{\circ} C$

Answer 2

$\frac{P^{\circ}_A - P_A}{P_A} = \frac{m \times M_A}{1000}$
$\frac{760 - 7 32}{732} = \frac{m \times 18}{1000}$
$m = 2.125$
$\Delta T_b = K_b m$
$= 0.52 \times 2.125$
$= 1.10$
$T_s = 100 + 1.10$
$= 101.1^{\circ} C$