Phosphoric acid is a triprotic acid (Ka1 = 6.9 x 10-3, Ka2 = 6.2x 10-8, and Ka3 = 4.8 x10-13). To find the pH of a buffer composed of H2PO4- (aq) and HPO42- (aq), which pKa value would you use in the Henderson-Hasselbalch equation?
Calculate the pH of a buffer solution obtained by dissolving 13.0 g of KH2PO4(s) and 26.0 g of Na2HPO4(s) in water and then diluting to 1.00 L.
We’re being asked to calculate the pH of buffer composed of H2PO4-(aq) and HPO42-(aq). To calculate for pH, we’re going to use the Henderson-Hasselbalch equation:
We know that
So first have to figure out which Ka value we’re going to use for our calculation. Phosphoric acid (H3PO4) is a weak triprotic acid, meaning it can donate three protons (H+) and it will have three equilibrium reactions.
Removing one H+ per step in H3PO4: