Neutral, Acidic, or Basic Salt

Question

Determine if the following salt is neutral, acidic or basic. If acidic or basic, write the appropriate equilibrium equation for the acid or base that exists when the salt is dissolved in an aqueous solution. If neutral, simply write only NR. CaCl₂

How would I go about determining the identity of the salt? What are the steps in writing an equilibrium equation?

Answer 1

To determine if the salt is neutral, acidic or basic, determine how the salt was made, or from which acid and base it was derived. For example, NaCl was derived from NaOH + HCl. KCN was derived from KOH + HCN. (Note: the cation of the salt comes from the base and the anion comes from the acid). Once you've determined how the salt was made, determine if the acid and base are both strong, weak or one of each. If both are strong (e.g. NaOH + HCl), then the salt (NaCl) is NEUTRAL. If the base is strong and the acid is weak (KOH + HCN), then the salt (KCN) is BASIC. IF the base is weak and the acid is strong (NH₃ + HCl), then the salt (NH₄Cl) is ACIDIC. If both the base and the acid are weak (NH₃ + CH₃COOH), then then salt (CH₃COONH₄) will have a pH determined by which is stronger (the acid or the base) and this will depend on the Ka of the weak acid compared to the Kb of the weak base.

The steps in writing the equilibrium equation involve writing the hydrolysis reaction, i.e. reacting the ion from the weak acid or base with water. There will be no such hydrolysis if the salt is neutral. Two examples are shown below.

1). For a strong base + weak acid salt (KCN). Here we look at the hydrolysis of CN^-.

CN^- + H₂O ==> HCN + OH^- This is the equilibrium equation

2). For a strong acid + weak base salt (NH₄Cl). Here we look at the hydrolysis of NH₄⁺.

NH₄⁺ + H₂O ==> NH₃ + H₃O⁺ This is the equilibrium equation