Question

An amount of solid $NH_4HS$ is placed in a flask already containing ammonia gas at a certain temperature and 0.50 atm. pressure. Ammonium hydrogen sulphide decomposes to yield $NH_3$ and $H_2S$ gases in the flask. When the decomposition reaction reaches equilibrium, the total pressure in the flask rises to 0.84 atm. The equilibrium constant for $NH_4HS$ decomposition at this temperature is

• A. 0.30
• B. 0.18
• C. 0.17
• D. 0.11

Answer 1

Answer: (D)

$NH_4HS (s) \rightleftharpoons NH_3(g) + H_2S (g)$
If $x$ is the partial pressure of $H_2S$ formed at equilibrium.
$(p_{NH_3})_{eqm} = (0.50 + x) atm$
$(p_{H_2})_{eqm} = x \,atm$
$(p_{NH_3})_{eqm} + (p_{H_2S})_{eqm} = 0.50 + x + x = 0.84$
$x 0.17$ atm
$K_p = (p_{NH_3})_{eqm} \times (p_{H_2s})_{eqm}$
$= (0.50 + 0.17) \times 0.17 = 0.1139 = 0.11$