How many moles of benzoic acid, a monoprotic acid with Ka=6.4x10^-5, muct be dissolved in 500 mL water to?

Question

produce a solution with pH=2.5?

Answer 1

First convert the pH to a [H+]. That will also equal the benzoate ion concentration. Then, substitute those into the expression for Ka and solve for the [benzoic acid]. That will give you an answer in moles/L, so multiply that by 0.5 to get the moles to dissolve in 500 mL. Finally, convert the moles into grams with the molar mass of benzoic acid.