Question

You will use two of the following materials to create a buffer solution. Determine the pKa for all conjugate acid/base pairs that could be used to make a buffer. Concentrations are approximate.

.2 M sodium formate [Na(HCO2)]

.2 M potassium hydrogen phosphate [K2HPO4]

.2 M ammonium chloride [NH4Cl]

.2 M hydrochloric acid

.2 M sodium hydroxide

Write all possible pairs of these five reagents, which might result in a buffer. List the pKa of each possible buffer mixture.

Hint: There are four mixtures that could work

Answer 1

pKa for,

0.2 M sodium formate [Na(HCO2)] = 3.75

0.2 M potassium hydrogen phosphate [K2HPO4] = 7.20

0.2 M ammonium chloride [NH4Cl] = 4.74

0.2 M hydrochloric acid : -7

0.2 M sodium hydroxide : 15.7

Combination of buffers possible

0.2 M ammonium chloride [NH4Cl] and 0.2 M sodium hydroxide [NaOH] pKa 4.74

0.2 M sodium formate [Na(HCO2)] and 0.2 M NaOH [NaOH] pKa 3.75

0.2 M potassium hydrogen phosphate [K2HPO4] and 0.2 M NaOH pKa 12.35

0.2 M potassium hydrogen phosphate and 0.2 M HCl pKa 7.20