Question

What weight of glucose (mol. wt. $=180)$ should be added to 1700 gof water at ${20}^{\circ }C$ to lower its vapour pressure by 0.001$mm$ ? The vapour pressure ofwater at ${20}^{\circ }C$ is 17$mmHg$ .

Answer 1

Applying formula
$\frac{p^o-P_5}{p^o}=x_B\Rightarrow$ mole fraction of solute
$\frac{0.001}{17}=\frac{no.ofmolesofglucose}{totalmoles}$
$\frac{0.001}{17}=\frac{\frac{x}{180}}{\frac{x}{180}+\frac{1700}{18}}$
$\frac{0.001}{17}=\frac{\frac{x}{180}}{\frac{x+17000}{180}}\Rightarrow 0.001x+17=17x$
$17=17x-0.001x$
$x=\frac{17}{16.99}$
$x=1g$ ans.