Question

Phosphoric acid is a triprotic acid (Ka1 = 6.9× 10–3, Ka2 = 6.2× 10–8, and Ka3 = 4.8× 10–13). 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?

A) pKa1= 2.16

B) pKa2=7.21

C) pKa3= 12.32

Calculate the pH of a buffer solution obtained by dissolving 24.0 g of KH2PO4(s) and 43.0 g of Na2HPO4(s) in water and then diluting to 1.00 L.

Answer 1

First, the equations

H3PO4 <> H2PO4- + H+ Ka1

H2PO4- <> HPO4-2 + H+ Ka2

HPO4-2 <> PO4-3 +H+ Ka3

since we need H2PO4- and HPO4-2 choose

H2PO4- <> HPO4-2 + H+ Ka2

therefore

answer is B pKa2 = 7.21

b)

m = 24 g of KH2PO4

MW = 136.086

mol = mas/MW = 24/136.086 = 0.1763

m = 43

MW = 141.96

mol = mass/MW = 43/141.96 = 0.3029

since it is 1 L these can be consideres as cconcnetrations

pH = pKa2 + log(A-/HA)

pH = 7.21 + log(0.1763/0.3029) = 6.9749