Question

What is the ratio of lactic acid (Ka = 1.37x10-4) to lactate in a solution with pH =4.49?

Answer 1

When Ka =1.37 x 10^-4 

∴Pka = - ㏒Ka

          = -㏒ 1.37 x 10^-4 
          
           = 3.86

According to H- H equation: 

when PH = PKA + ㏒[conjugate base / weak acid]

when the lactate is the conjugate base and lactic acid is the weak acid 

∴ PH = Pka +㏒[lactate] / [lactic acid]

so, by substitution:


4.49 = 3.86 + ㏒[lactate]/[lactic acid]

∴[lactate]/[lactic acid] = 4.3

∴ [lactic acid] / [lactate] = 1/4.3 
            
                                        = 0.23