a) Here, H2PO4- Acts as acid and HPO42- as salt. In equation pKa of acid is used, Hence. pKa = 7.21 b) pH = pKa + log (salt/acid) moles of salt = 30/142 moles of acid = 17/136 pH = 7.21 - 0.228 = 6.982 Solution a) Here, H2PO4- Acts as acid and HPO42- as salt. In equation pKa of acid is used, Hence. pKa = 7.21 b) pH = pKa + log (salt/acid) moles of salt = 30/142 moles of acid = 17/136 pH = 7.21 - 0.228 = 6.982.