Ammonium sulphide and ammonium selenide on heating dissociate as:

$(\text{NH}_4)_2\text{S}(s) \rightleftharpoons 2\text{NH}_3(g) + \text{H}_2\text{S}(g)$; $K_{p1} = 9 \times 10^{-3}$ atm$^3$

$(\text{NH}_4)_2\text{Se}(s) \rightleftharpoons 2\text{NH}_3(g) + \text{H}_2\text{Se}(g)$; $K_{p2} = 4.5 \times 10^{-3}$ atm$^3$

The total pressure over the solid mixture at equilibrium is