Calculate the pH of the resultant solution when solutions of acids and/or bases are diluted or mixed

Problem Statement:

6mL 0.15M NaOH solution is added to 15mL 0.15M CH3COOH solution. Calculate pH of the resulting solution [pKa = 4.74]


The reaction between CH3COOH and NaOH is as follows

From the above reaction, we can see that 1 mole of NaOH can neutralize the same amount of CH3COOH to produce 1 mole of CH3COONa.
Therefore, 6mL 0.15M NaOH can neutralize 6mL 0.15M CH3COOH to produce 6mL 0.15M CH3COONa

Now, remaining acid = (15 – 6)mL = 9mL
Produced salt = 6mL
Total volume of solution = (15 + 6)mL = 21mL