Question

The vapour pressure of water is 17.54 mm Hg at 293 K. Calculate vapour pressure of 0.5 molal solution of a solute in it.(Round off the answer).

Answer 1

molality = $\frac{n_{solute}}{m_{solvent}}$
here, molality = 0.5
so, mole of solute 0.5 mole per kg of water
now using relative lowering as $\frac{n}{n+N}$
1 kg of water is 55.55 moles
$\frac{0.5}{0.5+55.55}$
so relative lowering is 0.16
$17.54-0.16=17.38$
Ans-17.38