Question

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

A(*) ⇋ B(g) + D(g); Kp1 = 400

C(*) ⇋ E(g) + D(g); Kp2 = 900

At 25 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 Kp, 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(*) ⇋ B(g) + D(g); Kp1 = 400 p1 p1

C(*) ⇋ E(g) + D(g); Kp2 = 900 p2 p2

Kp1 = PB . PD = P1 (P1 + P2 ) = 400 . . . (i)

and KP2 = PE . PD = 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.