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How do I tackle Kc questions?

Kc is the equilibrium dissociation constant for an equilibrium, measured under standard conditions, and gives information about the extent of reaction, for any reversible reaction. 

The question regarding Kc, can be tricky at times. Following steps provide an easy method to solve Kc questions:

  1. For each of the reactant and product, write the initial moles, change in moles and equilibrium moles. Donate unknown with x.

  2. The amount of moles of product formed are linked to moles of reactant used, based on stoichiometric ratio. Use this to write equations for equilibrium moles of reactants and products. For instance, let x be moles of product formed:

                                             A       +     B     ⇌        C      +      D

Initial moles:                        a                b                0              0

Final moles:                         a-x            b-x              x              x

  1. Substitute the value of a and b.

  2. Substitute the expressions in the Kc formula and solve to find the value of x.

  3. Use the value of x to calculate equilibrium moles.

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