Henderson–Hasselbalch equation is given as,
pH = pKa + log [A⁻] / [HA] -------- (1)
Solution:
Convert Ka into pKa,
pKa = -log Ka
pKa = -log 1.37 × 10⁻⁴
pKa = 3.863
Putting value of pKa and pH in eq.1,
4.29 = 3.863 + log [lactate] / [lactic acid]
Or,
log [lactate] / [lactic acid] = 4.29 - 3.863
log [lactate] / [lactic acid] = 0.427
Taking Anti log,
[lactate] / [lactic acid] = 2.673
Result:
2.673 M lactate salt when mixed with 1 M Lactic acid produces a buffer of pH = 4.29.