Answer:
pH = 8.0
Explanation:
Initially, we need to determine the moles of NaOH.

Consider the balanced reaction.
C₂H₄O₃ + NaOH ⇒ C₂H₃O₃Na + H₂O
The molar ratio among C₂H₄O₃, NaOH, and C₂H₃O₃Na is 1: 1: 1. Therefore, if 7.2 × 10⁻⁴ moles of NaOH completely react with 7.2 × 10⁻⁴ moles of C₂H₄O₃, they will yield 7.2 × 10⁻⁴ moles of C₂H₃O₃Na.
To find the concentration of C₂H₃O₃Na, we note that:

C₂H₃O₃Na dissociates as follows:
C₂H₃O₃Na(aq) ⇒ C₂H₃O₃⁻(aq) + Na⁺(aq)
The anion C₂H₃O₃⁻ originates from a weak acid, which thus goes through basic hydrolysis.
C₂H₃O₃⁻ + H₂O ⇄ C₂H₄O₃ + OH⁻
Given that the pKa for C₂H₄O₃ is 3.9, we can derive pKb for C₂H₃O₃⁻ using this relation:
pKa + pKb = 14
pKb = 14 - 3.9 = 10.1
10.1 = -log Kb
Kb = 7.9 × 10⁻¹¹
We can compute [OH⁻] through the equation:
[OH⁻] = √(Kb.Cb) where Cb indicates the base's initial concentration
[OH⁻] = √(7.9 × 10⁻¹¹ × 0.012M) = 9.7 × 10⁻⁷ M
We can then find pOH and pH.
pOH = -log [OH⁻] = -log (9.7 × 10⁻⁷) = 6.0
pH + pOH = 14
pH = 14 - pOH = 14 - 6.0 = 8.0