Question

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.

Answer 1

Answer:The ratio of the concentrations of $HA$  and $A^-$ when the buffer has a pH of 7.02 is 0.69

Explanation:

The dissociation constant for formic acid = $K_a=1.8\times 10^{-4}$

Concentration of HA= 0.5 mM

Concentration of $A^-$= 0.1 mM

pH = 6.16

First we have to calculate the value of $K_a$.

Using Henderson Hesselbach equation :

$pH=pK_a+\log \frac{[A^-}{[HA]}$

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

$6.16=pK_a+\log (\frac{[0.1]}{0.5})$

$pK_a=6.86$

To calculate the ratio of the concentrations of $HA$  and $A^-$ when the buffer has a pH of 7.02.

Using Henderson Hesselbach equation :

$7.02=6.86+\log \frac{[A^-]}{[HA]}$

$7.02=6.86+\log \frac{[A^-]}{[HA]}$

$\frac{[A^-]}{[HA]}=1.44$

$\frac{[HA]}{[A^-]}=0.69$

Thus the ratio of the concentrations of $HA$  and $A^-$ when the buffer has a pH of 7.02 is 0.69