Question

Calculate the equilibrium concentration of undissociated CH3CHOHCOOH in a lactic acid solution with an analytical lactic acid concentration of 0.0767 and a pH of 3.130.

Answer 1

Let us write the equilibrium

CH3CHOHCOOH <=====> H+ + CH3CHOHCOO¯

Let us write the expression for Ka

Ka = ( [H+] [CH3CHOHCOO¯] ) / [CH3CHOHCOOH]

Since it is mono protic acid, there will be 1:1 ratio between  [H+] and [CH3CHOHCOO¯]

Let us calculate the hydronium ion concentration using pH

pH = - Log H+

3.13 = -Log H+

H+ = 7.41 x 10-4 M

Ka = [7.41 x 10-4] [7.41 x 10-4] / [ 0.0767 -7.41 x 10-4]

Ka = 7.22 x 10-6

Hence the equilibrium concentration of lactic acid solution is 7.22 x 10-6