Lactic acid is a weak acid found in milk. Its calcium salt is a source of calcium for growing animals. A saturated solution of this salt, which we can represent as (Ca(Lact)2) has a [Ca2+]=0.26M and a pH=8.40.
Calculate the Ka of lactic acid, assuming the salt is 100% dissociated.

Ka of lactic acid is 8.24 x 10⁻⁴

Explanation:

Given-

[Ca²⁺] = 0.26M

pH = 8.4

Salt is 100% dissociated

Ka = ?

Ca(Lact)₂ = Ca²⁺ + 2 Lact-

[lact⁻ ] = 0.26 x 2

          = 0.52 M

pOH = 14 - 8.40

        = 5.60

[OH⁻] = 10⁻ X 60 = 2.51 x 10⁻⁶ M = [lact⁻]

Kb = Kw/Ka

     =

     = 1.21 x 10⁻¹¹

Ka = Kw/Kb

    =

    = 8.24 x 10⁻⁴

Thus, Ka of lactic acid is 8.24 x 10⁻⁴

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