Question

Assume you have 4 weak acids: HA(pKa=4), HB (pKa= 4.5), HC (pKa=4.8), HD (pKa=5.5). If you want to prepare a pH=4.7 buffer, what acid and its cesium salt are the best? If the concentration of the acid is 0.1 M, what is the concentration of the cesium salt?

Answer 1

To prepare a buffer solution of pH = 4.7, the pKa of weak acid should be almost equal t pH. Thus the acid HC (pKa=4.8) is most suitable.

HC have a pKa of 4.8 hence it is the best choice with its cesium salt

HC(aq) + CsOH(aq) -------> C-Cs(aq) + H2O(l)

If concentration of the acid is 0.1 M, the the concentration of the cesium salt can be calculated as

pH = pKa + log[salt]/[acid]

4.7 = 4.8 +logx/0.1

x = 0.08 M

Thus the concentration of the cesium salt = 0.08 M