A certain buffer is made by mixing a weak acid HA and its conjugate base A–. When HA is present at a concentration of 0.5 mM and A– is present at a concentration of 0.1 mM, the buffer has a pH of 6.16. Calculate the ratio of the concentrations of HA and A– when the buffer has a pH of 7.02.

The ratio of the concentrations of  and when the buffer has a pH of 7.02 is 0.69

Explanation:

The dissociation constant for formic acid =

Concentration of HA= 0.5 mM

Concentration of = 0.1 mM

pH = 6.16

First we have to calculate the value of .

Using Henderson Hesselbach equation :

Now put all the given values in this expression, we get:

To calculate the ratio of the concentrations of  and when the buffer has a pH of 7.02.

Using Henderson Hesselbach equation :

Thus the ratio of the concentrations of  and when the buffer has a pH of 7.02 is 0.69