The 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 $NH_2HS$ decomposition at this temperature is :
