An acetic acid buffer containing 0.50 M acetic acid (CH3COOH) and 0.50 M sodium acetate (CH3COONa) has a pH of 4.74. What will the pH be after 0.0020 mol of HCl has been added to 100.0 mL of the buffer

pH = 4.71

Explanation:

We can find the pH of a buffer (Mixture of weak acid: CH3COOH, and its conjugate base: CH3COONa) using H-H equation:

pH = pKa + log [CH3COONa] / [CH3COOH]

Where pH is the pH of the buffere = 4.74, pKa the pka of the buffer and [] could be taken as the moles of each reactant.

As initially [CH3COONa] = [CH3COOH], [CH3COONa] / [CH3COOH] = 1:

pH = pKa + log 1

4.74 = pKa

To solve this question we need to find the initial moles of each species, The CH3COONa reacts with HCl to produce CH3COOH. That means the moles of CH3COOH after the reaction are: Initial CH3COOH + Moles HCl

Moles CH3COONa: Initial CH3COONa - Moles HCl.

Moles CH3COOH:

0.100L * (0.50mol / L) = 0.050 moles CH3COOH + 0.0020 moles HCl =

0.052 moles CH3COOH

Moles CH3COONa:

0.100L * (0.50mol / L) = 0.050 moles CH3COONa - 0.0020 moles HCl =

0.048 moles CH3COONa

Using H-H equation:

pH = 4.74 + log [0.048 moles] / [0.052 moles]