Question

Which of the following combinations would be best to buffer an aqueous solution at a pH of 7.0?

a) H2PO4– and HPO42–, Ka2 = 6.2E-8

b) NH4+ and NH3, Ka = 5.7E-10

c) H3PO4 and H2PO4–, Ka1 = 7.5E-3

d) CH3CO2H and CH3COO–, Ka = 1.8E-5

e) HNO2 and NO2–, Ka = 4.5E-4

Answer 1

pH of a buffer made up of acid and conjugate base can be calculated by Henderson-Hasselbach equation:

pH=pKa+log([A−][HA])

where pH signifies the concentration of [H+],

pK_a is the acid dissociation constant, and

[A-] and [HA] are concentrations of the conjugate base and starting acid.

Now if we have to make buffer an aqueous solution at a pH of 7.0 from the following combinations

a) H2PO4– and HPO42–, Ka2 = 6.2E-8

b) NH4+ and NH3, Ka = 5.7E-10

c) H3PO4 and H2PO4–, Ka1 = 7.5E-3

d) CH3CO2H and CH3COO–, Ka = 1.8E-5

e) HNO2 and NO2–, Ka = 4.5E-4

The best choice will be H₂PO₄^– and HPO₄^2–, because,

Because pka for this combination is – log 6.2 X 10 ^-8 or 6.339.