Answer
(a) $K_{eq} = \frac{[Cl_2]}{[PCl_5]}$
(b) $K_{eq} = [O_2]^3$
(c) $K_{eq} = \frac{[F^-][H_3O^+]}{[HF]}$
(d) $K_{eq} = \frac{[NH_4^+][OH^-]}{[NH_3]}$
Work Step by Step
1. Write the fraction:
$K_{eq} = \frac{[Products]}{[Reactants]}$
Remember: Those expressions do not consider solids (s) or liquids (l).
2. Put the balancing coefficient as the exponent of the concentration of the compound the coefficient is in front of.