CaCl 2 ·6H 2 O(s) ⇌ CaCl 2 ·4H 2 O(s) + 2H 2 O(g) has K p = 2.5×10 -5 atm 2 at 25°C. The maximum pressure of water vapour in the atmosphere, below which the hexahydrate is efflorescent, is

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K p = (P H2O ) 2 = 2.5×10 -5 atm 2 P H2O = 5.0×10 -3 atm P = 5.0×10 -3 ×760 = 3.8 mm Hg Efflorescence occurs when the moisture pressure is below this value. This is option (b).

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