You need to prepare 100.0 mL of a pH 4.00 buffer solution using 0.100 M benzoic acid (pK = 4.20) and 0.180 M sodium benzoate.

You need to prepare 100.0 mL of a pH 4.00 buffer solution using 0.100 M benzoic acid (pK₁ = 4.20) and 0.180 M sodium benzoate. How many milliliters of each solution should be mixed to prepare this buffer?