Читать книгу Geochemistry - William M. White - Страница 139

Consider soil water with a pH of 7 containing a weak organic acid, which we will designate HA, at a concentration of 1 × 10⁻⁴ M. If the apparent dissociation constant of the acid is 10^−4.5, what fraction of the acid is dissociated?

Answer: We have two unknowns: the concentration of the dissociated and undissociated acid, and we have two equations: the equilibrium constant expression for dissociation and the mass balance equation. We will have to solve the two simultaneously to obtain the answer. Our two equations are:

Solving the dissociation constant expression for [A^–], we have:

Then solving the conservation equation for [HA] and substituting, we have

Setting H⁺ to 10⁻⁷ and ΣHA to 10⁻⁴, we calculate [A^–] as 3.16 × 10⁻⁵ M, so 31.6% of the acid is dissociated.