Question

When solid ammonium chloride dissociates at a certain temperature in a 0.500 dm3 container,
ammonia and hydrogen chloride are formed.
NH4Cl (s) NH3(g) + HCl (g)
The initial amount of ammonium chloride was 1.00 mol, and when the system had reached
equilibrium there was 0.300 mol of ammonium chloride.
What is the numerical value of Kc for this reaction under these conditions?

Answer 1

1) 0.500 dm3 = 0.5 liter

2) Equilibrium reaction


        NH4Cl (s) ⇄  NH3(g) + HCl (g)

3) Table of changes


Number of moles

Species              NH4Cl            NH3          HCl

Start                      1mol              0              0

Change                  - 0.7           + 0.7      +  0.7  

Equilibrium              0.3              0.7            0.7

 
4) Equilibrium constant


Kc = [NH3] [HCl]


[NH3] = 0.7 mol / 0.5 liter =  1.4 M

[HCl] = 0.7 mol / 0.5 liter = 1.4 M


Kc = (1.4M)^2 = 1.96M^2


The numerical value is just 1.96 (without the units)