Question

What is the ratio of lactic acid (ka = 1.37x10-4) to lactate in a solution with ph =4.12?

Answer 1

we will use the PH formula:
PH = Pka + ㏒ [salt/acid] 
 PH= Pka  +㏒ [lactate/lactic]
when Pka = -㏒Ka
                 = -㏒ 1.37x10^-4
                 = 3.86
and we have PH =4.12
So by substitution:
4.12 = 3.86 + ㏒[actate/lactic]
∴[actate/lactic] = 1.82
∴[lactic/actate] = 1 / 1.82
                         = 0.55