The strength of a base is measured by its tendency to attract a proton.

The strongest base possible in aqueous solution is the hydroxide ion. If a base stronger than OH^- is dissolved in water (e.g. oxide ion), it reacts instantly with the water molecules to form hydroxide ions. For this reason, the hydroxide ion is considered to be "the" strong base.

Whereas hydronium ions are formed from an ionization reaction with water, hydroxide ions are formed from dissociation when an ionic compound dissolves in water. To find the hydroxide concentration of an ionic hydroxide solution, assume that the ionic compound dissociates completely upon dissolving.

Example 1

Find the hydroxide concentration of a 0.01 mol/L solution of NaOH.

\( \mathrm { NaOH(aq) \rightarrow Na^+(aq) + OH^- (aq)~~~Assume~complete~dissociation } \)

1 mole of NaOH completely dissociates in water to form 1 mole of Na⁺ ions and 1 mole of OH^- ions.

Therefore, the concentration of hydroxide ions in a 0.01 mol/L solution of NaOH(aq) is 0.01 mol/L.

Example 2

Consider a 0.064 mol/L solution of barium hydroxide.

\( \mathrm { Ba(OH)_2 (aq) \rightarrow Ba^{2+}(aq) + 2OH^- (aq)~~~Assume~complete~dissociation } \)

1 mole of Ba(OH)₂ completely dissociates in water to form 1 mole of Ba²⁺ ions and 2 moles of OH^- ions.

Therefore, the hydroxide ion concentration of a 0.064 mol/L solution of barium hydroxide equals the molarity of the original barium hydroxide solution multiplied by 2/1.

\( \mathrm { 0.064 \frac{mol}{L} \times \dfrac{2}{1} = 0.13 \frac{mol}{L} } \)

The hydroxide concentration of the barium hydroxide solution is 0.13 mol/L.

Molecular and ionic compounds other than hydroxides dissolve in water to produce basic solutions. These compounds are referred to as "weak bases". The weak bases attract protons less strongly than do hydroxide ions and react only partially with water to form hydroxide ions (usually much less than 50%).

For example, consider the reaction of the weak base, CN^- with water.

On the Table of Relative Strengths of Acids and Bases, you will notice that base strength decreases as you move upwards on the table.