The vapour pressure of water over an aqueous solution of at is 186.8 torr. Given that the mole fract — Solutions and Colligative Properties Chemistry Question
Question
The vapour pressure of water over an aqueous solution of $\text{NH}_3$ at $70^\circ\text{C}$ is 186.8 torr. Given that the mole fraction of $\text{NH}_3$ in the solution at equilibrium is 0.20 and the vapour pressure of pure $\text{H}_2\text{O}(l)$ at $70^\circ\text{C}$ is 233.5 torr, determine whether the solution is ideal or not. Answer ‘1’, if the solution is ideal and answer ‘2’, if the solution is non-ideal.
💡 Solution & Explanation
If the solution is ideal, it obeys Raoult's law for the solvent: $P_{\text{H}_2\text{O}} = P_{\text{H}_2\text{O}}^\circ \times X_{\text{H}_2\text{O}}$. The mole fraction of water is $1 - 0.20 = 0.80$. Expected vapour pressure = $233.5 \text{ torr} \times 0.80 = 186.8 \text{ torr}$. This exactly matches the given observed vapour pressure, confirming the solution behaves ideally. Therefore, correct answer is 1.