Maths Skill: Calculating pH (AQA A Level Biology): Revision Note
Exam code: 7402
Calculating pH
If the hydrogen ion (H+) concentration of a solution is known, the pH can be calculated using the equation:
pH = -log₁₀ [H⁺]
You can find the ‘log’ function on your calculator (‘log’ is the same as ‘log10’ so don’t worry if the calculator doesn’t say ‘log10’)
Worked Example
The hydrogen ion concentration of a solution is 1.6 x 10-4 mol dm-3. Find the pH of this solution.
The pH of the solution is:
pH = -log₁₀ [H⁺]
pH = -log₁₀ 1.6 x 10-4 = 3.796
pH = 3.8
Worked Example
The hydrogen ion concentration of a solution of sodium hydroxide is 3.5 x 10-11 mol dm-3. Find the pH of this solution.
The pH of the solution is:
pH = -log₁₀ [H⁺]
pH = -log₁₀ 3.5 x 10-11 = 10.456
pH = 10.5
Worked Example
Ethanoic acid (also known as acetic acid) is a weak acid produced by wood ants that they can spray at predators as a defence mechanism. The hydrogen ion concentration of a sample of ethanoic acid taken from some wood ants was 8.39 x 10-6 mol dm-3. Find the pH of the ethanoic acid produced by wood ants.
The pH of the solution is:
pH = -log₁₀ [H⁺]
pH = -log₁₀ 8.39 x 10-6 = 5.076
pH = 5.08
Examiner Tips and Tricks
Don’t forget the minus sign in the formula: pH = –log₁₀ [H⁺] as this is a common exam mistake. pH values should be between 0 and 14. If your answer falls outside this range, check your working. Use clues in the question to estimate if your answer is reasonable (e.g. alkalis should have high pH, weak acids around pH 4–6).
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