'n' moles of a non electrolyte are added to 'N' moles of solvent. The addition causes a lowering in vapour pressure of solvent by 20%. The ratio of moles of solvent and its solute is :

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Solution

The expression for the relative lowering of vapor pressure is $\frac{P^{0} - P}{P^{0}} = \frac{n}{n + N}$
The lowering in vapor pressure of the solvent is 20%.
Hence, $\frac{100 - 80}{100} = \frac{n}{n + N}$
$\frac{20}{100} = \frac{n}{n + N}$
$20 n + 20 N = 100 n$
$20 N = 80 n$
$\frac{N}{n} = \frac{80}{20} = 4$
Thus, the ratio of the moles of the solvent and solute is 4.

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'n' moles of a non electrolyte are added to 'N' moles of solvent. The addition causes a lowering in vapour pressure of solvent by 20%. The ratio of moles of solvent and its solute is :

The expression for the relative lowering of vapor pressure is P0−PP0=nn+NThe lowering in vapor pressure of the solvent is 20%.Hence, 100−80100=nn+N20100=nn+N20n+20N=100n20N=80nNn=8020=4Thus, the ratio of the moles of the solvent and solute is 4.