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

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