Question

Calculate the mass (g) of NaCI that has to be dissolved to reduce the vapour pressure of 100 g of water by $10\%$ (Molar mass $NaCl=58.5gmol^{- 1}$ ).(Round off your answer to the nearest integer.)

Answer 1

The mass (g) of NaCI can be calculated from Raoult's law
$P_{s}=P^{0}\times $ mole fraction of solvent
$90 = 100 \times \left(\frac{100 / 18}{\frac{100}{18} + \frac{w \times 2}{58 . 5}}\right)$
$0 . 9 \left(\frac{100}{18} + \frac{w \times 2}{58 . 5}\right) = \frac{100}{18}$
$\Rightarrow w=18.11\sim eq18gm$