Answer to Question #179311 in Physical Chemistry for A Hinds

Question

Planning and design

You are required to conduct an experiment that should be buffered at pH = 5.2. The only chemicals available are butanoic acid and sodium butanoate. Plan and design an experiment to demonstrate how you would prepare 100 ml of this buffer using 10 ml of 0.05 M butanoic acid. Show all calculations.

Answer 1

This question deals with blood and cytoplasm, which can be considered as aqueous solutions containing many different solute species. These biological fluids are known to use a carbonic acid-bicarbonate ion buffer system. Carbonic acid is the weak acid and bicarbonate ion is the weak conjugate base. They are related by the following dissociation equilibrium at 25 degrees Celsius:

$H_2CO_3⇋H^++HCO_3^{−}$

$Ka=4.3×10^{−7}=[H^+]×\frac{[HCO_3^{−}]}{[H_2CO_3]}$

The buffer solution maintains its pH close to the pKa of the weak acid, which here will be:

$pKa=−log(K_a)=−log(4.3×10^{−7})=6.4$

This is below our target solution pH of 7.2. Therefore, we would need an excess of bicarbonate ion for our preparation.