Question

A buffer solution is made by mixing a weak acid with its conjugate base. If the ratio of conjugate base to acid is 4, and the pH of the buffer is 7.2, what is the pKa of the weak acid? Round the answer to one decimal place.

Answer 1

Answer:

6.6 is the $pK_a$ of the weak acid.

Explanation:

To calculate the pH of acidic buffer, we use the equation given by Henderson Hasselbalch:

$pH=pK_a+\log(\frac{[salt]}{[acid]})$

We are given:

$pK_a$ = negative logarithm of acid dissociation constant =?

The ratio of conjugate base to acid is = $\frac{[salt]}{acid}=4$

pH = 7.2

Putting values in above equation, we get:

$7.2=pK_a+\log(4)$

$pK_a=7.2-\log(4)=6.598\approx 6.6$

6.6 is the $pK_a$ of the weak acid.