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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry, 4th edition.

Problem 14.56.

Ammonium hydrogen sulfide dissociates into ammonia gas and hydrogen sulfide gas. If we start with a sample of pure NH4HS(s) at 25 oC, the total pressure of the gases is 0.658 atm when equilibrium is established. Write an equation for the dissociation reaction and determine the value of Kp




Answer:

Set up a table:
NH4HS(s)
NH3(g)
+
H2S(g)
Kp = PNH3(e)PH2S(e)
Initial
?
0
0
Change
–x
+x
+x
Equilibrium
?–x
x
x

In this case x represents the partial pressure of the gases in the container and the total pressure in the container is equal to the sum of the partial pressures (Dalton's Law of Partial Pressures). Thus, 0.658 atm = x + x = 2x, or x = 0.329 atm.

Now, Kp = (x)(x) = x2 = (0.329)2 = 0.108.