Question

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?

Answer 1

a) Ka2 is the most important

b) The pH of the buffer is 7.41

Step-by-step 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