AQA A Level Biology復習筆記1.4.9 Maths Skill: Calculating pH
Maths Skill: Calculating pH
- If the?hydrogen ion?(H+) concentration of a solution is known, the?pH?can be calculated using the equation:
pH?=?-log?? [H?]
- You can find the?‘log’?function on your calculator (‘log’ is the?same?as ‘log10’ so don’t worry if your 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 = -log?? [H?]
pH = -log?? 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 = -log?? [H?]
pH = -log?? 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 = -log?? [H?]
pH = -log?? 8.39 x 10-6?= 5.076
pH = 5.08
Exam Tip
Don’t forget the minus sign in the formula:?pH = -log?? [H?].?This is easy to overlook and is a common mistake that students make in exams. Remember: pH must fall between 0 and 14 so if your answer is outside of this range, something has gone wrong! Also, as seen in worked examples above, the question may give you a clue as to what your answer should roughly be. For example, we know that sodium hydroxide is an alkali, so a pH of 10.5 makes sense and we are told that the ethanoic acid produced by wood ants is a weak acid, so 5.08 makes sense too!
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