Acids and Bases



Strong bases

A strong base is one which is able to completely dissociate into its ions when in water. Therefore, in water is has to be a stronger base than OH.

A strong base in water is very rare because water is itself is a weak acid and so does not give up its protons readily to another species. One of the only common examples is the oxide ion.

The hydroxide ion is, in itself, quite stable and found in many ionic substances. If one of these substances is soluble then it is able to dissociate completely in aqueous solution. Although they do not react directly with the water, because they completely dissociate giving OH ions they are considered to be strong bases. For example, NaOH is a strong base.

An alkaline substance is one which dissolves in water to produce an excess of OHions.

Calculating pH

If you known the concentration of the solution then it is possible to work out the pH of a strong base.

For example: 0.1M NaOH: [OH] = 0.1, therefore [H3O+] = Kw/[OH] = Kw/0.1 = 1 x 10-13 M

So, the pH of 0.1M NaOH = 13.3

In this example the presence of OHdue to the auto-ionisation of water has been ignored.

The alkalinity of the solution above is limited only by the solubility of the hydroxide as it fully dissociates at all concentrations due to the fact that it is an ionic compound. At room temperature the maximum pH possible is 14.5 (in correspondence to a solution of approximately 3M NaOH).

Weak bases

A weak base, on the other hand, does not dissociate fully in water. However, for your exam you will not need to carry out weak base pH calculations.