Q63 / 180 · NEET 2020 (Phase 2) · Chemistry

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 molar mass of n-octane is $114 g {mol}^{- 1}$ ]

Correct answer
A
$40$
B
$60$
C
$80$
D
$20$

Solution

Assuming dilute solution,

\frac{P_{0} - P_{s}}{P_{s}} \approx \frac{P_{0} - P_{s}}{P_{0}} = \frac{n_{solute}}{n_{solvent}}

Let

P_{0} = 100,

V.P reduced to

80 %, ∴ P_{s} = 80

\frac{100 - 80}{100} = \frac{8 / m}{114 / 114}

m = 40

g/mol.

Hence, the correct answer is option: 'A'.

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