Hosphoric acid is a triprotic acid ( K a1 = 6.9 × 10 − 3 Ka1=6.9×10−3 , K a2 = 6.2 × 10 − 8 Ka2=6.2×10−8 , and K a3 = 4.8 × 10 − 13 Ka3=4.8×10−13 ). To find the pH of a buffer composed of H 2 PO − 4 ( aq ) H2PO4−(aq) and HPO 2 − 4 ( aq ) HPO42−(aq) , which p K a Ka value should be used in the Henderson–Hasselbalch equation? p K a1 Ka1 = 2.16 p K a2 Ka2 = 7.21 p K a3 Ka3 = 12.32 Calculate the pH of a buffer solution obtained by dissolving 15.0 15.0 g of KH 2 PO 4 ( s ) KH2PO4(s) and 31.0 31.0 g of Na 2 HPO 4 ( s ) Na2HPO4(s) in water and then diluting to 1.00 L. p H = pH=