Question

When solid ammonium chloride dissociates at a certain temperature in a 0.500 dm3 container, ammonia and hydrogen chloride are formed. 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? NH4Cl(s) ---> NH3(g) + HCl(g) I got 3.27 but the answer is 1.96.

Answer 1

1) Reaction

NH4Cl(s) ---> NH3(g) + HCl(g)


2) equilibrium equation, Kc


Kc = [NH3] * [HCl]


3) Table of equilibrium formation



step               concentrations
                      
                            NH4Cl(s)                     NH3(g)             HCl(g)


start                      1.000 mole                     0                    0


react                        - x


produce                                                       +x                  + x
                         ------------------                ----------              -----------

end                         1 - x                             +x                    +x
            

1 - x = 0.3 => x = 1 - 0.3 = 0.7


[NH3] = [HCl] = 0.7/0.5 liter = 1.4                (I used 0.500 dm^3 = 0.5 liter)


4) Equilibrium equation:


Kc = [NH3] [HCl] = (1.4)^2  = 1.96


Which is the number that you were looking for.


Answer: Kc = 1.96