Question

Two solids A and C dissociate into gas products as follows;

A(s) ⇋ B(g) + D(g); K_p1 = 400

C(s) ⇋  E(g) + D(g); K_p2 = 900

At 25º C, the pressure over excess solid A only is 400 atm, and that over solid C only is 60 atm. Find the pressure over solid mixture. 

Answer 1

Arrange the stepwise conversion and using the given values of K_p, frame for partial pressure and solve for solid mixture pressure. 

• When two solids A and C are taken together in a closed container, both decompose to give gases B, D and E.

• As D is the common gas, the dissociation of both the solids A and C shall be suppressed.

• Suppose that the partial pressures of B and D due to dissociation of only A are p₁ atm each, and the partial pressures of E and D due to dissociation of only C are p₂ atm each.

A(s) ⇋ B(g) + D(g); K_p1 = 400 _p1 p1 

C(s) ⇋  E(g) + D(g); K_p2 = 900 p2 p2

Kp₁ = P_B . P_D = P₁ (P₁ + P₂ ) = 400 . . . (i)

and K_P2 = P_E . P_D = P₂ (P₁ + P₂ ) = 900 . . . (ii)

Solving equation. (1) and (2),

p₁ = 11.097 and p₂ = 24.96 atm.

∴ pressure over solid mixture = 2(P₁ + P₂)

= 2(11.097 + 24.96) = 72.114 atm.