According to Henderson–Hasselbalch Equation,
pH = pKa + log [Lactate] / [Lactic Acid]
As,
Ka of Lactic Acid = 1.38 × 10⁻⁴
pKa = -log Ka
pKa = -log 1.38 × 10⁻⁴
pKa = 3.86
So,
pH = 3.86 + log [0.10] / [0.13]
pH = 4.74 + log 0.769
pH = 4.74 - 0.11
pH = 4.63
