This is an example calculation about the phosphate buffer system from my Biochemistry textbook. 1. The problem statement, all variables and given/known data If the total cellular concentration of phosphate is 20 mM (millimolar) and the pH is 7.4, the distribution of the major phosphate species is given by pH = pKa + log10 [HPO42-] / [H2PO4-] 7.4 = 7.20 + log10 [HPO42-] / [H2PO4-] [HPO42-] / [H2PO4-] = 1.58 Thus, if [HPO42-] + [H2PO4-] = 20 mM, then [HPO42-] = 12.25 mM and [H2PO4-] = 7.75 mM 2. Relevant equations pH = pKa + log10 [A-] / [HA] pH = -log10 [H+] 3. The attempt at a solution I understand everything up until they provide the concentrations of each phosphate species. Since their ratio as shown in the equation is 1.58, one can clearly assume that [HPO42-] > [H2PO4-]. But the fact that no explanation is provided for arriving at their specific concentrations is driving me insane. The Henderson-Hasselbalch equation shows that, when [HPO42-] / [H2PO4-] = 1, pH = pKa. But since we are at pH = 7.4, they obviously can't be equal. I think the solution must involve taking the 0.2 difference into account somehow.