AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Basic buffer solutions contain a weak base and the salt of that weak base.
Full specification wording
Buffer action (A-level only): Basic buffer solutions contain a weak base and the salt of that weak base.
A basic buffer is a weak base with the salt of that base, the classic pair being aqueous ammonia with ammonium chloride.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A basic buffer is a weak base with the salt of that base, the classic pair being aqueous ammonia with ammonium chloride. The salt supplies the ammonium ions and the base supplies the ammonia molecules, so both halves of the conjugate pair NH₄⁺ and NH₃ are present in quantity and the solution settles above pH 7. Added acid is removed by the base: NH₃ + H⁺ → NH₄⁺. Added alkali is removed by the salt: NH₄⁺ + OH⁻ → NH₃ + H₂O. The pairs that do not work are worth recognising. Two weak bases such as ammonia and methylamine are not a conjugate pair; two weak acids are not either; and a strong acid mixed with a strong base simply gives a neutral salt solution with nothing left over to absorb further additions.
What earns the marks
- Identifying a mixture of a weak base with its salt, such as ammonia with ammonium chloride.
- The equation NH₃ + H⁺ → NH₄⁺ for the removal of added acid.
- The equation NH₄⁺ + OH⁻ → NH₃ + H₂O for the removal of added alkali.
- Rejecting a pair of two weak acids, or a strong acid with a strong base, as not forming a buffer.
Common mistakes and how to put them right
Choosing two weak bases such as ammonia and methylamine, which are not a conjugate pair.
Choosing hydrochloric acid with sodium hydroxide, which gives only a neutral salt solution.
Writing NH₄OH as the species present in aqueous ammonia.
Using OH⁻ rather than the ammonia molecule as the species that removes added acid.
Stating that a basic buffer holds the pH at exactly 7.
Exam tips
- In a multiple-choice buffer question, look for two species that differ by one H⁺; that is the conjugate pair.
- Write ionic equations for basic buffers, since NH₃ and NH₄⁺ are the species that actually react.
Try the lesson
The start of the Buffer action (A-level only): Basic buffer solutions contain a weak base and the salt of that weak base. lesson, made from checked teaching notes for this specification point.
Have a go first
Which pair of substances, when mixed in aqueous solution, forms a basic buffer?
Listen
A basic buffer is a weak base with the salt of that base, the classic pair being aqueous ammonia with ammonium chloride. The salt supplies the ammonium ions and the base supplies the ammonia molecules, so both halves of the conjugate pair NH₄⁺ and NH₃ are present in quantity and the solution settles above pH 7. Added acid is removed by the base: NH₃ + H⁺ → NH₄⁺. Added alkali is removed by the salt: NH₄⁺ + OH⁻ → NH₃ + H₂O. The pairs that do not work are worth recognising. Two weak bases such as ammonia and methylamine are not a conjugate pair; two weak acids are not either; and a strong acid mixed with a strong base simply gives a neutral salt solution with nothing left over to absorb further additions. A strong answer earns marks for these points. identifying a mixture of a weak base with its salt, such as ammonia with ammonium chloride. the equation NH₃ + H⁺ → NH₄⁺ for the removal of added acid. the equation NH₄⁺ + OH⁻ → NH₃ + H₂O for the removal of added alkali. rejecting a pair of two weak acids, or a strong acid with a strong base, as not forming a buffer. Exam tip: In a multiple-choice buffer question, look for two species that differ by one H⁺; that is the conjugate pair.
Key words
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basic buffer
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weak base and its salt
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ammonia and ammonium chloride
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conjugate pair recognition
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See every definition
- basic buffer
- A solution that contains a weak base and the salt of that weak base.
- weak base and its salt
- The mixture of a weak base with the salt of that weak base, which forms a basic buffer.
- ammonia and ammonium chloride
- The classic pair, where the salt supplies ammonium ions and the base supplies ammonia molecules.
- conjugate pair recognition
- Looking for two species that differ by one H⁺, which is the conjugate pair.
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