How many moles of benzoic acid, a monoprotic acid with Ka=6.4×10^-5, must dissolve in 250mL of H2O to produce a solution with pH=2.04

Answer 1

Call benzoic acid HA, then
.....HA --> H^+ + A^-
E.....x.....

pH = -log(2.04)
Ka = (H^+)(A^-)/HA)
You know H^+. A^- is the same. You know Ka. Solve for x = HA. I get approximately 1.6 mols/L. For 250 mL you would need 1/4 that amount.