Step 1: Find the mole fraction of water. The solution has a mole fraction of solute =0.02. Mole fraction of water =1 minus mole fraction of solute. So, χ‌water ‌=1−0.02=0.98 Step 2: Use Raoult's Law to relate vapour pressures. Raoult's law says: Vapour pressure of solution = Vapour pressure of pure water × Mole fraction of water. p‌solution ‌=p‌water ‌×χ‌water ‌ We know p‌solution ‌‌‌=34.65mmHg and χ‌water ‌=0.98. So, 34.65=p‌water ‌×0.98 Step 3: Solve for the vapour pressure of pure water. Divide both sides by 0.98 : p‌water ‌=‌