Strong bases
A strong base completely converts into OH- ions in water. Common strong bases include sodium hydroxide and potassium hydroxide.
\(NaOH_{(s)} + (aq) ⟶ Na^+_{(aq)} + OH^-_{(aq)} \)
Calculating pH of strong bases using Kw
\(K_w = [H^+][OH^-] \)
At room temperature, Kw is assumed to be 1.00 x 10-14, 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 OH- ions.
2. Use Kw to work out the concentration of H+ ions.
3. Convert [H+] into pH.
For example: Calculate the pH of 0.1 mol dm-3 sodium hydroxide.
NaOH dissociates completely, so [OH-] = 0.1 mol dm-3.
\(1.00 × 10^{-14} = [H^+][0.1] \)
\([H^+] = \dfrac{1.00 × 10^{-14}}{0.1} = 1.00 × 10^{-13} \)
\(pH = - \log_{10}{(1.00 × 10^{-13})} = 13 \)
Weak bases
A weak base does not fully convert into OH- 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 OH- ions.
Note: The calculation of pH for a weak base is beyond the scope of all A-Level syllabuses.
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