At what relative humidity will Na₂SO₄·10H₂O(s) be efflorescent when exposed to the air at 27°C? — Chemical Equilibrium Chemistry Question

💡 Solution & Explanation

Efflorescence occurs when the partial pressure of water in the air is lower than the equilibrium vapour pressure of the hydrate. For Na₂SO₄·10H₂O, the equilibrium P_H₂O = 2 × 10⁻³ atm. Relative humidity = (Equilibrium P_H₂O / Saturated Vapour Pressure) × 100 = (2 × 10⁻³ / 0.04) × 100 = (0.002 / 0.04) × 100 = 5%. The salt will effloresce if relative humidity is below 5%. Therefore, correct answer is D.