AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
+ pH = –log [H ] Students should be able to: convert concentration of…
Full specification wording
Definition and determination of pH (A-level only): + pH = –log [H ] Students should be able to: convert concentration of hydrogen ions into pH and vice versa calculate the pH of a solution of a strong acid from its concentration.
For a strong monoprotic acid the dissociation is complete, so every mole of acid delivers a mole of H⁺ and [H⁺] is simply the acid concentration.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
For a strong monoprotic acid the dissociation is complete, so every mole of acid delivers a mole of H⁺ and [H⁺] is simply the acid concentration. Then pH = −log₁₀[H⁺]: for 0.0500 mol dm⁻³ hydrochloric acid, pH = −log(0.0500) = 1.30. Working backwards, [H⁺] = 10⁻ᵖᴴ, so a solution of pH 3.40 has [H⁺] = 3.98 × 10⁻⁴ mol dm⁻³. Two traps recur. A diprotic strong acid supplies two protons per molecule, so 0.0200 mol dm⁻³ H₂SO₄ gives [H⁺] = 0.0400 mol dm⁻³ and pH = 1.40. And if the acid has been diluted or mixed, recalculate the concentration from the moles and the new total volume before taking the logarithm.
What earns the marks
- Stating that [H⁺] equals the acid concentration for a strong monoprotic acid.
- Substituting correctly into pH = −log₁₀[H⁺].
- The pH quoted to two decimal places.
- Using [H⁺] = 10⁻ᵖᴴ when converting a pH back to a concentration.
- Doubling the concentration when the strong acid is diprotic.
Common mistakes and how to put them right
Using a number of moles rather than a concentration in mol dm⁻³ inside the logarithm.
Forgetting that 0.100 mol dm⁻³ H₂SO₄ provides 0.200 mol dm⁻³ H⁺.
Giving the pH to a whole number when two decimal places are expected.
Using Ka for a strong acid, which is unnecessary because dissociation is complete.
Failing to recalculate the concentration after dilution before taking the logarithm.
Exam tips
- Write [H⁺] on its own line before you touch the log button; most lost marks happen before the calculator, not on it.
- For 'concentration from pH', use the 10ˣ key with a negative exponent and keep three significant figures.
Try the lesson
The start of the Definition and determination of pH (A-level only): + pH = –log [H ] Students should be able to: convert concentration of hydrogen ions into pH and vice versa calculate the pH of a solution of a strong acid from its concentration. lesson, made from checked teaching notes for this specification point.
Have a go first
A strong monoprotic acid solution has a hydrogen ion concentration of 0.001 mol dm⁻³. What is the pH of this solution?
Listen
For a strong monoprotic acid the dissociation is complete, so every mole of acid delivers a mole of H⁺ and [H⁺] is simply the acid concentration. Then pH = −log₁₀[H⁺]: for 0.0500 mol dm⁻³ hydrochloric acid, pH = −log(0.0500) = 1.30. Working backwards, [H⁺] = 10⁻ᵖᴴ, so a solution of pH 3.40 has [H⁺] = 3.98 × 10⁻⁴ mol dm⁻³. Two traps recur. A diprotic strong acid supplies two protons per molecule, so 0.0200 mol dm⁻³ H₂SO₄ gives [H⁺] = 0.0400 mol dm⁻³ and pH = 1.40. And if the acid has been diluted or mixed, recalculate the concentration from the moles and the new total volume before taking the logarithm. A strong answer earns marks for these points. stating that [H⁺] equals the acid concentration for a strong monoprotic acid. substituting correctly into pH = −log₁₀[H⁺]. the pH quoted to two decimal places. using [H⁺] = 10⁻ᵖᴴ when converting a pH back to a concentration. Exam tip: Write [H⁺] on its own line before you touch the log button; most lost marks happen before the calculator, not on it.
Key words
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strong acid pH
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complete dissociation
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logarithm and antilogarithm
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diprotic acids
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See every definition
- strong acid pH
- For a strong monoprotic acid the dissociation is complete, so every mole of acid delivers a mole of H⁺ and [H⁺] is simply the acid concentration, with pH = −log₁₀[H⁺].
- complete dissociation
- For a strong monoprotic acid every mole of acid delivers a mole of H⁺, so [H⁺] is simply the acid concentration.
- logarithm and antilogarithm
- pH = −log₁₀[H⁺] converts concentration of hydrogen ions into pH, and working backwards [H⁺] = 10⁻ᵖᴴ converts pH into concentration.
- diprotic acids
- A diprotic strong acid supplies two protons per molecule, so 0.0200 mol dm⁻³ H₂SO₄ gives [H⁺] = 0.0400 mol dm⁻³ and pH = 1.40.
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