Under most cases, you will be using the equilibrium constant K_c, which value is equal to

K_c = [C]^c[D]^d

      ----------

      [A]^a[B]^b

The letters in square brackets represent chemical concentrations at equilibrium, and

the powers that they are raised to are their stoichiometric coefficients.

I.e. the reaction is aA+ bB -> cC + dD

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The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium,