Question

The vapour pressure of pure water at ${60}^{o}C$ id 150 torr. The vapour pressure of water over a solution at ${60}^{o}C$ containing equal numbers of moles of water and ethylene glycol (a non-volatile solute) is 50 torr. Which of the following is correct?

• A. The relative lowering of vapour pressure = $\frac{2}{3}$
• B. The soluation is ideal according to Raoult's law
• C. The solution has positive deviation from ideal behavior
• D. $\Delta H_{mix}<0$

Answer 1

Answer: (A)

The relative lowering of vapour pressure = $\frac{2}{3}$

$P^0$ solvent $=150$ torr
$P_s=50$ torr
Relative lower $y=\frac{P_{solvent}^0-Ps}{P_{solvent}^0}$
$=\frac{150-50}{150}$
$\frac{100}{150}=\frac{2}{3}$