pH of a mixture of 1 M benzoic acid (pKa, = 4.20) and 1 M sodium benzoate is 4.5. In 300 mL buffer, benzoic acid is

If benzoic acid = x mL x millimoles sodium benzoate=(300−x)mL=(300−x) millimoles  pH=pKa+log⁡ [sodium benzoate]  [benzoic acid]  4.5=4.2+log⁡300−xx 300−xx=2 x = 100 mL