Step 1:

A buffer solution contains  0.300 mol of HC₂H₃O₂.

And the buffer solution also contains  0.300 mol of NaC₂H₃O₂.

pH of a buffer solution is 4.74. \ \

Formula for pH is .

Use the formula to evaluate the pK_a.

So the pK_a  of acetic acid = 4.74.

Step 2:

Now find the pH when 0.020 mol NaOH is added the reaction.

C₂H₄CO₂ + OH^- = C₂H₃CO₂^- + H2O

Moles acetic acid = 0.300 - 0.020 =0.280.

Moles acetate = 0.300 + 0.020=0.320.

Now find the pH.

Therefore the pH when 0.020 mol NaOH is added is 4.80. \ \

Solution:

pH when 0.020 mol NaOH is added is 4.80.