Answer
(a) $K_{eq} = \frac{[H_3O^+][CHO_2^-]}{[HCHO_2]}$
(b) $K_{eq} = \frac{[HCO_3^-][OH^-]}{[CO_3^{2-}]}$
(c) $K_{eq} = \frac{[CO]^2}{[O_2]}$
(d) $K_{eq} = \frac{[CO]^2}{[CO_2]}$
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.