A buffer consists of 0.21 M NaH2PO4 and 0.34 M Na2HPO4. Phosphoric acid is a triprotic acid (Ka1 = 7.2 * 10-3, Ka2 = 6.3 * 10-8, Ka3 = 4.2 * 10-13). (a) Which Ka value is most important to this buffer? Ka1 is most important. Ka2 is most important. Ka3 is most important. (b) What is the buffer pH?

a) Ka2 is the most important

b) The pH of the buffer is 7.41

Explanation:

Step 1: Data given

Molarity of NaH2PO4 = 0.21 M

Molarity of Na2HPO4 = 0.34 M

a) Which Ka value is most important to this buffer?

Phosphoric acid will ionize in aqueous solution according to the following equilibrium reactions

H3PO4 (aq) + H2O (l) → H2PO4 - (aq) + H3O + (aq) Ka1 = 7.2*10^-3

H2PO4 - (aq) + H2O (l) → HPO42 - (aq) + H3O + (aq) Ka2 = 6.3 * 10^-8

HPO42 - (aq) + H2O (l) → PO43 - (aq) + H3O + (aq) Ka3 = 4.2 * 10^-13

Ka2 is the most important constant, since it involves the equilibrium between the 2 phosporic acid salt species you have. (H2PO4 - and HPO42-)

b) What is the buffer pH?

H2PO4 - (aq) + H2O (l) → HPO42 - (aq) + H3O + (aq)

H2PO4 - (aq) → HPO42 - (aq) + H+

pH = pKa + log [conjugate base]/[acid]

pH = - log (6.3 * 10^-8) + log (0.34/0.21)

pH = 7.2 + 0.21

pH = 7.41

The pH of the buffer is 7.41