bStrong bases/b

A strong base ucompletely/u converts into OHsup-/sup ions in water. Common strong bases include sodium hydroxide and potassium hydroxide.

(NaOH_(s) + (aq) ⟶ Na^+{(aq)} + OH^-{(aq)} )

bCalculating pH of strong bases using Ksubw[/sub]/b

(K_w = [H^+]OH^- )

At room temperature, Ksubw/sub is assumed to be 1.00 x 10sup-14/sup, therefore:

(1.00 × 10^-14 = [H^+]OH^- )

In order to calculate the pH of a strong base, the steps are:

1. Work out the concentration of the OHsup-/sup ions.
2. Use Ksubw/sub to work out the concentration of Hsup+/sup ions.
3. Convert Hsup+/sup into pH.

For example: Calculate the pH of 0.1 mol dmsup-3/sup sodium hydroxide.

NaOH dissociates completely, so OHsup-/sup = 0.1 mol dmsup-3/sup.

(1.00 × 10^-14 = [H^+]0.1 )

(H⁺ = \dfrac{1.00 × 10^-14}{0.1} = 1.00 × 10^{-13} )

(pH = - \log₁₀{(1.00 × 10^{-13})} = 13 )

bWeak bases/b

A weak base does not fully convert into OHsup-/sup ions in water. A common example is ammonia.

(NH_{3 (aq)} + H_2O_{(l)} ⇌ NH_{4 (aq)}^+ + OH^-_{(aq)} )

The majority of the ammonia exists as ammonia molecules. Only a minority has produced OHsup-/sup ions.

iNote: The calculation of pH for a weak base is beyond the scope of all A-Level syllabuses./i