Brønsted-Lowry acids, pH and Kw
At A-level an acid is defined by what it does with protons. pH is a logarithmic scale for the concentration of hydrogen ions, and the ionic product of water lets you find the pH of alkalis as well as acids.
Part 1 of 3: Learn it
In short
- A Brønsted-Lowry acid is a proton donor; a base is a proton acceptor.
- pH = −log₁₀[H⁺], and [H⁺] = 10^(−pH).
- Kw = [H⁺][OH⁻] = 1.00 × 10⁻¹⁴ mol² dm⁻⁶ at 298 K.
Where this is in your specification
Spec points: AQA 7405 3.1.12.1 to 3.1.12.3, OCR A H432 5.1.3, Edexcel 9CH0 Topic 12
| Board | Topic: Acids, bases and pH |
|---|---|
| AQA 7405 | 3.1.12 |
| Edexcel 9CH0 | Topic 12 |
| OCR H432 | 5.1.3 |
Acids and bases
In HCl + H₂O → H₃O⁺ + Cl⁻, HCl donates a proton (H⁺) and water accepts it. A strong acid such as HCl or HNO₃ dissociates completely in water; a weak acid such as ethanoic acid only partly dissociates. Strong and weak describe how much it dissociates, not how concentrated it is.
The pH scale
A change of one pH unit is a tenfold change in [H⁺]. For a strong monoprotic acid, [H⁺] equals the acid's concentration. Give pH values to 2 decimal places.
Kw and strong bases
- Water ionises slightly: H₂O ⇌ H⁺ + OH⁻.
- Kw = [H⁺][OH⁻] = 1.00 × 10⁻¹⁴ mol² dm⁻⁶ at 298 K. It increases with temperature, because the ionisation is endothermic.
- For a strong base, [OH⁻] equals its concentration (times 2 for Ba(OH)₂). Then [H⁺] = Kw ÷ [OH⁻], and pH follows.
- Pure water is neutral because [H⁺] = [OH⁻], even when its pH is not exactly 7 at other temperatures.
A solution has pH 3.40. Find [H⁺].
Show the answer
10^(−3.40) = 3.98 × 10⁻⁴ mol dm⁻³.
Part 2 of 3: See it worked
Worked examples
Example 1
Find the pH of 0.050 mol dm⁻³ hydrochloric acid.
- Strong acid, so [H⁺] = 0.050 mol dm⁻³
- pH = −log₁₀(0.050)
Answer: 1.30.
Example 2
Find the pH of 0.020 mol dm⁻³ sodium hydroxide at 298 K.
- [OH⁻] = 0.020 mol dm⁻³
- [H⁺] = 1.00 × 10⁻¹⁴ ÷ 0.020 = 5.0 × 10⁻¹³ mol dm⁻³
- pH = −log₁₀(5.0 × 10⁻¹³)
Answer: 12.30.
Common mistakes
- Using ln instead of log₁₀.
- Calling a dilute strong acid weak.
- Forgetting that Ba(OH)₂ gives two hydroxide ions per formula unit.
- Saying water at 50 °C is acidic because its pH is below 7. [H⁺] still equals [OH⁻], so it is neutral.
Define a Brønsted-Lowry base.
Show the answer
A proton (H⁺) acceptor.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
A solution has pH 3.40. Find [H⁺].
10^(−3.40) = 3.98 × 10⁻⁴ mol dm⁻³.
Define a Brønsted-Lowry base.
A proton (H⁺) acceptor.
By what factor does [H⁺] change when pH falls from 5 to 3?
It increases by a factor of 100.
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