Ammonium sulphide and ammonium selenide on heating dissociates as (NH 4 ) 2 S(s) ⇌ 2NH 3 (g)+ H 2 S(g); K p1 = 9 × 10 -3 atm 3 (NH 4 ) 2 Se(s) ⇌ 2NH 3 (g) + H 2 Se(g); K p2 = 4.5 × 10 -3 atm 3 The total pressure over the solid mixture at equilibrium is

Step-by-step solution

NH 4 2 S s ⇌ 2NH 3 g + H 2 S g 2p p NH 4 2 Se s ⇌ 2NH 3 g + H 2 Se g 2q q K p 1 = p NH 3 2 · p H 2 S = 2 p + 2 q 2 · p = 9.0 × 10 - 3 K p 2 = p NH 3 2 · p H 2 S e = 2 p + 2 q 2 · q = 4.5 × 10 - 3 ∴ p q = 2 ⇒ p = 2 q ∴ K p 1 = 2p + p 2 · p = 9.0 × 10 - 3 ∴ p = 0.1 atm and q = 0.1 2 = 0.05 Total pressure = 2p + 2q + p + q = 3p + 3q = 3 × 0.1 + 3 × 0.05 = 0.45 atm