Ionic Equilibrium - Calculating constants.

In the GCE "A" level syllabus, we are concern with three type of ionic equilibria, and they are acid dissociation constant, K_a, base dissociation constant, K_b and solubility product K_sp. These constants, K, are all equilibrium constants, where the volume of solution is constant. In addition, we are required to perform calculation to obtain the different equilibrium concentration.

(A) Acid Dissociation Constant.

A weak acid dissociates partially in water. Hence, the weak acid dissociates in a reversible equation thus an equilibrium is obtained. The experiment set up is depicted below.

Using the equilibrium equation, we can create an ICE table (ICE = Intial, Change and Equilibrium) to determine the initial concentrations of the substances found in the equilibum, the change in concentration of the substances and the equilibrium concentration of these substances. We can do a direct substraction/addition between two concentrations because the volume of solution is the same (as shown in the above diagram).

(B) Base Dissociation Constant.

The weak base dissociates partially, hence a weak base dissociates in a reversible equation thus an equilibrium is obtained. The set up is similar to that of a weak acid. Like in calculating the Acid Dissociation Constant, a similar ICE table is drawn. It is shown in the picture below. Do note that the concentration of water is not the variable as it is the medium where the solute dissolves in.

(C) Solubility Product.

This is applicable for ionic compounds that you learn are insoluble in water. Ionic compounds which are soluble in water will fully dissociate in water into its constituent ions. However, because of partial solubility due to interaction between solvent molecules and solid, this results in some amount of the ionic compound to dissolve in water. Hence, as a result despite a solid is insoluble in water, there are still traces of its constituent ions found in solution.

The diagram set up and the calculation is show below.

As the ionic compound is a solid, concentration of a solid is not a variable.

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Article written by Kwok YL 2009.}

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