Weak bases react with strong acids to form acidic salt solutions. The conjugate acid of the weak base determines its pH. For example, NH3 (ammonia) is added to HCl (hydrochloric acid) to form NH4Cl (ammonium chloride).NH3(aq) + HCl(aq) → NH4Cl(aq) weak strong salt base acid
As soon as the salt is formed it reacts with water, resulting in a slightly acidic solution.