Question

Challenge: The Kp for the decomposition of NH4HS is 0.126 at 25oC.

NH4HS (s) ⇌ NH3 (g) + H2S (g)

If a student combines 0.500 atm of ammonia with 0.500 atm of hydrogen sulfide in a 1.00L flask at 25 oC, how many grams of ammonium hydrogen sulfide will be formed?

Answer 1

Kp = P-NH3 * P-H2S

ignore solids in Kp

we are actually forming reverse reaction.

NH3 (g) + H2S (g) ⇌ NH4HS (s)

Kp = 1/Kp = 1/0.126 = 7.94

initially

P-NH3 = 0.5

P-H2S = 0.5

in equilibrium

P-NH3 = 0.5 - x

P-H2S = 0.5 - x

7.94 = 1/(P-H2S *P-NH3)

7.94 = 1 / ( 0.5 - x)^2

sqrt(7.94 ) = 1/(0.5-x)

2.817*(0.5-x) = 1

2.817*0.5 - 2.817x = 1

-2.817x = 1-2.817*0.5

x = -0.4085/-2.817 = 0.14501

then

P-H2S = 0.5 - x = 0.5-0.14501

P-H2s = 0.35499 mol left

mol of NH4HS formed = 0.14501

mass = mol*MW = 0.14501*51.11 = 7.41 g of NH4HS