In a galvanic cell the cathode is an Ag+ (1.00 M) / Ag(s) half-cell. The anode is a standard hydrogen electrode immersed in a buffer solution containing 0.10 Mbenzoic acid (C6H5COOH) and 0.050 M sodium benzoate (C6H5COO-Na+). The measured cell voltage is 1.030 V. What is the pKa of benzoic acid?