The pH scale

Nature of science:

Occam’s razor—the pH scale is an attempt to scale the relative acidity over a wide range of H+ concentrations into a very simple number. (2.7)

• pH = − log[H⁺(aq)] and [H⁺] = 10^−pH.
• A change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H⁺] .
• pH values distinguish between acidic, neutral and alkaline solutions.
• The ionic product constant, Kw = [H⁺] [OH^-] = 10⁻¹⁴ at 298 K.

Applications and skills:

• Solving problems involving pH, [H⁺] and [OH^-].
• Students should be familiar with the use of a pH meter and universal indicator.

Guidance:

• Students will not be assessed on  values.
• Students should be concerned only with strong acids and bases in this sub- topic.
• Knowing the temperature dependence of Kw  is not required.
• Equations involving H₃O⁺  instead of H may be applied.

• Chemistry makes use of the universal language of mathematics as a means of communication. Why is it important to have just one “scientific” language?

Utilization:

• Syllabus and cross-curricular links:
• Mathematics SL (topic 1.2)and Mathematics HL (topic 1.2)—study of logs

Aims:

• Aim 3: Students should be able to use and apply the pH concept in a range of experimental and theoretical contexts.
• Aim 6: An acid–base titration could be monitored with an indicator or a pH probe.