Question

If 8 g of a non-electrolyte solute is dissolved in 114 g of n-octane to reduce its vapour pressure to 80%, the molar mass (in g mol-1) of the solute is
[Given that the molar mass of n-octane is 114 g mol-1.]

• A. $40$
• B. $60$
• C. $80$
• D. $20$

Answer 1

Answer: (A)

$40$

Answer 2

Let us take a dilute solution,
$\frac{P_{0}-P_{s}}{P_{s}} \approx\frac{P_{0}-P_{s}}{P_{0}}=\frac{n_{\text{Solute}}}{n_{\text{Solvent}}}$
Let $P_0$ = 100,
Vapour Pressure reduced to 80%
$\therefore P_{s}=80$

$\frac{100-80}{100}=\frac{8 m}{114}$
$m=40$

Therefore, the correct option is (A) : 40.