If CuSO4.5H2O(s)  → CuSO4.3H2O(s) + 2H2O(l)  ; Kp = 1.086 × 10^–4 atm2 at 25°C. The efflorescent  nature of CuSO4.5H2O can be noticed when vapour pressure of H2O in atmosphere is

Q:  If CuSO4.5H2O(s)  $ \rightleftharpoons $ CuSO4.3H2O(s) + 2H2O(l)  ; Kp = 1.086 × 10–4 atm2 at 25°C. The efflorescent  nature of CuSO4.5H2O can be noticed when vapour pressure of H2O in atmosphere is

(A) > 7.29 mm

(B) < 7.92 mm

(C)  ≥ 7.92 mm

(D) None

Solution:   An efflorescent salt is one that loses H2O to atmosphere.

For the reaction

CuSO4.5H2O(s)  $ \rightleftharpoons $ CuSO4.3H2O(s) + 2H2O(l)

Kp = (p’H2O)2 = 1.086 × 10–4

p’H2O = 1.042 × 10–2 atm = 7.92 mm

If p’H2O at 25°C < 7.92 mm only then, reaction will proceed in forward direction.

Ans: (B)