# AQA A Level Biology复习笔记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 = -log₁₀ [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 = -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

#### Exam Tip

Don’t forget the minus sign in the formula: pH = -log₁₀ [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! 