Question

A lactic acid/lactate ion buffer solution contains 0.14 M HC3H5O3 and 0.83 M C3H5O3−, respectively. The Ka value of lactic acid is 1.4×10−4. Calculate the pH of this buffer.

Answer 1

Use Henderson Hasselbache
PH=pka+log([salt]/[acid])
Pka=-log(ka)
-log(1.4x10^-4)= 3.85
log(.83/.14)= .77
3.85+.77= 4.62
PH=4.62

Answer 2

Answer: The pH of the buffer is 4.62

Explanation:

The chemical reaction for the hydrolysis of lactic acid follows the equation:

$HC_3H_5O_3+H_2O\rightleftharpoons C_3H_5O_3^-+H_3O^+$

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

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

$pH=pK_a+\log(\frac{[C_3H_5O_3^-]}{[HC_3H_5O_3]})$

We are given:

$pK_a$ = negative logarithm of acid dissociation constant of lactic acid = 3.85

$[C_3H_5O_3^-]=0.83M$

$[HC_3H_5O_3]=0.14M$

pH = ?

Putting values in above equation, we get:

$pH=3.85+\log(\frac{0.83}{0.14})\\\\pH=4.62$

Hence, the pH of the buffer is 4.62