Dissociation Constant or Ionization Constant for Weak

Acids & BasesWhen a weak acid, HX, is placed into water some of it reacts with water to form H3O+ and X- ions. An equilibrium is established, in which the dissociation reaction HX + H2O H3O+ + X- occurs at the same rate as the association reaction, H3O+ + X- HX + H2O.

We describe this situation by writing the reaction equation as:

HX(aq) + H2O(l) H3O+(aq)+ X-

(aq)

When the law of chemical equilibrium is applied, the mass action expression we get is:

Keq = [H3O+] [X-] [HX]

Since Keq expresses the tendency for the acid to dissociate, it is often called the “dissociation constant” or “ionization constant” for the weak acid. symbolically: Kdiss or Ka weak acids

Kdiss or Kb weak bases

NOTE:

1. If Ka, Kb, Kdiss > 10: The substance is essentially 100% dissociated for our purposes. This will happen with strong acids & bases like HCl or NaOH.

2. If Ka, Kb, Kdiss < 10: Dissociation is less extensive & the acid or base is said to be weak.

3. A knowledge of Kdiss for a weak acid or base is useful to chemists since it allows the [H3O+] & pH of the solution to be calculated.

Ka Problems – Type IFinding the % dissociation & Kdiss given concentrationIf the pH of a 0.100 M solution of CH3COOH is 2.86, find

a) % diss

b) Ka

Type II: Finding concentrations, % dissociation when given Ka or Kdiss

Given a 0.900 M solution of a weak acid, HX, whose Ka is 4.5 x 10-4, find the [H3O+], pH & % dissociation.

Type II with a twistFind the [H3O+] & pH of a solution which was made up to be 0.100 M HOCN (cyanic acid) & 0.0500 M in NaOCN (sodium cyanate). Assume complete dissociation. Ka = 3.47 x 10-4