How to Calculate pH of Weak Base

In this JC2 webinar we want to learn how to calculate the pH of a weak base.

The method is very similar to finding the pH of a weak acid.

Let's take a look at the question.

calculate pH of weak base question

Since ammonia is a weak base, it will be partially dissociated.

We can work out the concentrations of all the species at equilibrium via the ICE table.

calculate pH of weak base ICE table

We can then substitute these values into the base dissociation constant (Kb) expression.

In order to solve for x, we need to impose an approximation to avoid solving quadratic equation.

We will assume that the concentration of weak base at equilibrium is equal to its original concentration.

We can then rearrange the equation to determine x, which is the concentration of OH-.

calculate pH of weak base substitute into Kb with approximation

Since we are dealing mostly with monoprotic species in Ionic Equilibria, the ICE table and approximation is always valid.

Therefore we can use the following formula to quickly determine OH- concentration when Kb and weak base concentration are given.

calculate pH of weak base formula

For this exercise Kb of ammonia is not given.

But we can get this easily from the Ka of its conjugate acid NH4+ and Kw, the ionic product of water.

calculate pH of weak base Ka Kb equals Kw

Please note Ka.Kb = Kw only applies to conjugate acid-base pairs.

We can now use the formula method to find OH- concentration.

calculate pH of weak base calculate hydroxide concentration

Finally we can determine pOH and pH of this weak base.

calculate pH of weak base calculate pOH and pH

Topic: Ionic Equilibria, Physical Chemistry, A Level Chemistry, Singapore

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