In an exactly. 200M solution of benzoic acid, a monoprotic acid, the [H+] = 3.55 X 10-3M. what is the Ka for benzoic acid?

Write the reaction. Ka is the product of the product's concentration raised to the power of the coefficients from the balanced equation divided by the product's of the reactants raised to the power of their coefficients. The concentration of the products will be equal since for every [H+] produced the concentration of the other product is the same. The concentration of the reactant is the original concentration minus what was converted to products.