AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Therefore, a logarithmic scale, the pH scale, is used as a measure of…
Full specification wording
Definition and determination of pH (A-level only): Therefore, a logarithmic scale, the pH scale, is used as a measure of hydrogen ion concentration.
The pH scale compresses that huge range by taking a logarithm: pH = −log₁₀[H⁺], with the concentration in mol dm⁻³.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The pH scale compresses that huge range by taking a logarithm: pH = −log₁₀[H⁺], with the concentration in mol dm⁻³. Because the logarithm is to base 10, one whole pH unit corresponds to a factor of ten in hydrogen ion concentration, and the minus sign makes acidic solutions, which have large [H⁺], have small pH values. Nothing restricts pH to the range 0 to 14: 2 mol dm⁻³ hydrochloric acid has a negative pH and concentrated sodium hydroxide exceeds 14. pH 7 is neutral only at 298 K, because neutrality means [H⁺] = [OH⁻] and the concentration at which that happens depends on Kw, which rises as temperature rises. Since a logarithm is being taken, answers are normally quoted to two decimal places.
What earns the marks
- pH = −log₁₀[H⁺] with the concentration in mol dm⁻³.
- Stating that a fall of one pH unit means a tenfold rise in [H⁺].
- Stating that a neutral solution has pH 7 only at 298 K.
- Quoting the pH to two decimal places.
Common mistakes and how to put them right
Using ln rather than log₁₀ on the calculator.
Omitting the minus sign, so the pH of an acid comes out negative.
Calling any solution of pH 7 neutral regardless of temperature.
Rounding pH to a whole number, when 0.1 of a pH unit is already a 26% change in [H⁺].
Exam tips
- Sanity-check every answer: an acid should give a small pH and an alkali a large one.
- Give pH to two decimal places unless the question says otherwise; whole numbers lose marks.
Try the lesson
The start of the Definition and determination of pH (A-level only): Therefore, a logarithmic scale, the pH scale, is used as a measure of hydrogen ion concentration. lesson, made from checked teaching notes for this specification point.
Have a go first
The pH scale is a logarithmic measure of hydrogen ion concentration. If the pH of an aqueous solution decreases by 2.0 units, how does the hydrogen ion concentration change?
Listen
The pH scale compresses that huge range by taking a logarithm: pH = −log₁₀[H⁺], with the concentration in mol dm⁻³. Because the logarithm is to base 10, one whole pH unit corresponds to a factor of ten in hydrogen ion concentration, and the minus sign makes acidic solutions, which have large [H⁺], have small pH values. Nothing restricts pH to the range 0 to 14: 2 mol dm⁻³ hydrochloric acid has a negative pH and concentrated sodium hydroxide exceeds 14. pH 7 is neutral only at 298 K, because neutrality means [H⁺] = [OH⁻] and the concentration at which that happens depends on Kw, which rises as temperature rises. Since a logarithm is being taken, answers are normally quoted to two decimal places. A strong answer earns marks for these points. pH = −log₁₀[H⁺] with the concentration in mol dm⁻³. stating that a fall of one pH unit means a tenfold rise in [H⁺]. stating that a neutral solution has pH 7 only at 298 K. quoting the pH to two decimal places. Exam tip: Sanity-check every answer: an acid should give a small pH and an alkali a large one.
See it
![A top-down flowchart. The first node reads 'Hydrogen ion concentration [H⁺] in mol dm⁻³' and points to 'Take logarithm to base 10', which points to 'Apply minus sign: pH = −log₁₀[H⁺]', which points to 'pH value'. From 'pH value' five arrows lead to: 'One pH unit = factor of ten in [H⁺]', 'Large [H⁺] gives small pH (acidic)', 'pH not restricted to 0 to 14', 'pH 7 neutral only at 298 K', and 'Quote pH to two decimal places'.](https://xnnrgnazirrqvdgfhvou.supabase.co/storage/v1/object/public/lesson-media/16cd4054-04be-4132-b379-e7a3eda128c6/diagram-299626e08db4.svg)
Key words
Say what each word means before you turn the card.
logarithmic scale
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pH definition
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factor of ten per unit
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neutrality and temperature
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See every definition
- logarithmic scale
- A scale used as a measure of hydrogen ion concentration, compressing a huge range by taking a logarithm.
- pH definition
- The negative logarithm to base 10 of the hydrogen ion concentration, with the concentration in mol dm⁻³.
- factor of ten per unit
- One whole pH unit corresponds to a factor of ten in hydrogen ion concentration, because the logarithm is to base 10.
- neutrality and temperature
- Neutrality means [H⁺] = [OH⁻], and the concentration at which that happens depends on Kw, which rises as temperature rises, so pH 7 is neutral only at 298 K.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
![The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first node reads 'Hydrogen ion concentration [H⁺] in mol dm⁻³' and points to 'Take logarithm to base 10', which points to 'Apply minus sign: pH = −log₁₀[H⁺]', which points to 'pH value'. From 'pH value' five arrows lead to: 'One pH unit = factor of ten in [H⁺]', 'Large [H⁺] gives small pH (acidic)', 'pH not restricted to 0 to 14', 'pH 7 neutral only at 298 K', and 'Quote pH to two decimal places'.](https://xnnrgnazirrqvdgfhvou.supabase.co/storage/v1/object/public/lesson-media/16cd4054-04be-4132-b379-e7a3eda128c6/diagram-labelled-299626e08db4.svg)
1Box 1
2Box 2
3Box 3
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