Let’s solve for a solution. Imagine we have a pure solvent (Water) with a vapour pressure of 100 mmHg. We add some salt so that the 'mole fraction' of the water is 0.8 (meaning 80% of the particles on the surface are water). Step 1: Identify the pure pressure (P° = 100 mmHg). Step 2: Identify the mole fraction of the solvent (X = 0.8). Step 3: Apply Raoult’s Law: P = P° × X. Step 4: Calculate: P = 100 × 0.8 = 80 mmHg. The new vapour pressure is 80 mmHg, which is lower than the original 100 mmHg.