A liquid shows a decrease in vapour pressure by 10 mm Hg. What will be the mole fraction of the solvent if the vapour pressure decreases by 20 mm Hg instead?
Using a formula, Raoult’s Law - Relative Lowering of Vapour Pressure $$
X_{\text{solvent}}= \frac {P_{0}- P} {P_{0}}
$$
Here,
Case 1- Decrease in vapour pressure by 10 mm Hg. So, the relative lowering =$$
X_{\text{solvent}}= \frac {100- 90}{100}=0.10
$$
Mole fraction of solute = 0.10
So, mole fraction of solvent = 1 − 0.10 = 0.90
Case 2- Decrease in vapour pressure by 20 mm Hg. So, the relative lowering =$$
X_{\text{solvent}}= \frac {100- 80}{100}=0.20
$$
Mole fraction of solute = 0.20
So, mole fraction of solvent = 1 − 0.20 = 0.80