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  4. The vapour pressure of water at 20 text℃ is 17.54 mm Hg. When 20 g of a non-ionic, substance is dissolved in 100 g of water, the vapour pressure is lowered by 0.30 mm Hg. What is the molecular mass of the substance?

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Q. The vapour pressure of water at $20\text{℃}$ is 17.54 mm Hg. When 20 g of a non-ionic, substance is dissolved in 100 g of water, the vapour pressure is lowered by 0.30 mm Hg. What is the molecular mass of the substance?

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Solution:

$\frac{\text{p} ^\circ - \text{p}_{\text{s}}}{\text{p} ^\circ }=\text{X}_{\text{s} \text{o} \text{l} \text{u} \text{t} \text{e}}$
or
$\frac{\text{p} ^\circ - \text{p}_{\text{s}}}{\text{P}_{\text{s}}}=\frac{\text{n}}{\text{N}}$
$\text{∴}\frac{0 .3}{17 .24}=\frac{\frac{20}{M}}{\frac{100}{18} + \frac{20}{M}} \, \left\{\right.M=molarmassofsolute\left.\right\}$
$So,M=207.087g/mol$