Answer
$$K_c = [O_2]^3$$
Work Step by Step
$$K_c = \frac{[Products]}{[Reactants]} = \frac{[O_2]^3}{1} = [O_2]^3$$
- The equilibrium expression for a chemical equation is equal to the multiplication of the concentration of the products divided by that of the reactants.
That expression does not include compounds in the solid or liquid state. In this case, $KClO_3(s)$ and $KCl(s)$ were not included for that specific reason.
Notice: The equilibrium coefficients of each molecule/ion will give the exponent. In that case, since the coefficient of $O_2$ is equal to 3, that will be the exponent of its concentration.
