pH (A2 Only)
Chemistry: AQA A Level, topic 2.5.2
pH
pH is a measure of the acidity of a solution.
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Definition
- The mathematical definition of pH is:
- pH = -log10[H+]
Rearranging the pH equation
- You can use the pH of a solution to calculate it's hydrogen ion concentration.
- According to the rules of logarithms, you can rearrange the above equation to:
- [H+] = 10-pH
Calculations of pH
- Consider a one molar solution of HCl. The [H+] = 1moldm-3
- pH = -log10(1) = 0
Uses of pH
- We use pH because the concentration of hydrogen ions varies massively over solutions.
- It can vary between 2 or 3 moldm-3 to 1 × 10-20 or so.
- This is a very big scale, so we use a logarithmic one to shorten it.
- You can have negative pH, but it’s rare to see one below -1 or above 15.
- This is a much smaller range, and much nicer to work with.
Test yourself on pH (A2 Only)
What is the mathematical definition of pH?
If we rearrange the pH equation to pH = -log10[H+]
to make [H+] the subject, what will the equation be?