# Weak Acids and Bases

Weak acids and bases cannot dissociate completely. They undergo the same type of dissociation as that of strong acids and bases, but the extent of dissociation is very little compared to strong acid or strong base dissociations.

# Acid-ionization Constant (Ka)

A typical way to represent the dissociation of a weak acid is shown below:

In this reaction, the protons from the weak acid are transferred to the water molecules. The acid-ionization constant (Ka) of this reaction is shown below:

We encounter weak acids every day. An example is carbonated water that contains dissolved carbon dioxide, and is called carbonic acid. Fruits like lemons and oranges contain citric acid which is also a weak acid. The list goes on and on.

# Percentage Ionization

Percentage ionization is the percentage expression of the degree of ionization. What is the degree of ionization of a weak acid? Degree of ionization is the fractional amount of the weak acid that gets ionized.

Example
Find the degree of ionization of 0.1 M solution of acetic acid (CH3COOH). Also find the pH of the solution.

(The acid-ionization constant of acetic acid is 1.8 x 10â€“5)

Solution

We will logically dissect the events that lead to the dissociation. At first we do not have any H+ and CH3COOâ€“ ions. Let's say we have a unit volume (a liter) of acetic acid and xM (mol/L) of it was ionized. We can represent the equation as follows:

With the concentrations at equilibrium we can substitute the concentrations in the acid-ionization constant expression.

Even though this has to be solved using a quadratic equation, we can skip that elaborate process by making some chemically acceptable assumptions. Since the acid-ionization constant is very small, we can assume the same with the value of x. So the expression changes as follows. For the MCAT, you won't be given problems that require extensive calculations. The problems will test mostly concepts, and when calculations are involved the numbers will usually be manageable ones.

Solving for x you will get roughly 1.34 x 10â€“3 = 0.00134 M.

The question also asks for pH of the solution.

pH = â€“ log [H+]

The hydrogen ion concentration is 0.00134 M. By substituting in the pH formula you should get the following result.

The pH of the solution is 2.87.

# Base-ionization Constant (Kb)

Just like the acid-ionization constant, there is also the base-ionization constant. Consider the ionization of ammonia in water.

For the above reaction, we can write the base-ionization constant (Kb) for the above reaction as follows: