Equilibria of Weak Bases, Kb

What is a Weak Base?

A weak base is any base that reacts with water (accepts H+ ions) to a very small extent, usually less than 5 - 10%.  An aqueous solution of a weak base in a state of equilibrium would consist mainly of the unionized form of the base, and only a small amount of hydroxide ions and of the cation (conjugate acid) of the weak base.  The equation representing the ionization of any weak acid, B, and the equilibrium expression, Kb, are shown below.

Equation and Equilibrium Expression for B

Note: This expression, Kb, is based on the general form for Kc. The designation Kb is used to indicate that it is the equilibrium constant for the reaction of a base with water.


Calculating Kb

To calculate the ionization constant, Kb, you need to know:

To find Kb substitute the values for the equilibrium concentrations into the equilibrium expression and solve for Kb.

Example:  The pH of a 0.1000 M solution of aniline solution was found to be 8.79.  Determine the value of  Kb, the ionization constant for aniline.  The formula for aniline is C6H5NH2.

Calculation of K<sub>b</sub >


Calculating the Equilibrium Concentrations in an Aqueous Solution of a Weak Base

To calculate the equilibrium concentrations you need to know:

To determine the equilibrium: Example:  Calculate the concentrations for each species present in a 0.1000 M aqueous solution of ammonia (Kb = 1.8 x 10-5).