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  4. The mass of a non-volatile solute of molar mass 60 g mol -1 that should be dissolved in 126 g of water to reduce its vapour pressure nto 99 % wil be

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Q. The mass of a non-volatile solute of molar mass $60\, g\, mol ^{-1}$ that should be dissolved in $126\, g$ of water to reduce its vapour pressure nto $99 \%$ wil be

COMEDKCOMEDK 2012Solutions

Solution:

Relative lowering in vapour pressure
= mole fraction of the non-volatile solute
$\frac{p^{\circ}-p_s}{p^{\circ}}=x_2=\frac{w_2 / M_2}{w_1 / M_1+w_2 / M_2}$
$\frac{100-99}{100}=\frac{w_2 / 60}{126 / 18+w_2 / 60}$
$w_2=4.2=4 \,g$