pH & pOH
Chemistry: IB Diploma Higher Level, topic 6.2.1
pH
pH is a measure of the acidity of a solution.
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Definition
- The mathematical definition of pH is:
- pH = -log10[H3O+]
- H+ (hydrogen ion) and H3O+ (hydronium ion) are often used interchangeably to describe pH in aqueous solution.
- While both are accepted, H3O+ is preferred. If you recall all H+ ions yield H3O+ ions in solution:
- H2O + H+ → H3O+.
Rearraging the pH equation
- You can use the pH of a solution to calculate it's hydronium ion concentration.
- According to the rules of logarithms, you can rearrange the above equation to:
- [H3O+] = 10-pH
Calculations of pH
- Consider a one molar solution of HCl. The [H3O+] = 1 mol L-1
- pH = -log10(1) = 0
Uses of pH
- We use pH because the concentration of hydronium ions varies massively over solutions.
- It can vary between 2 or 3 mol L-1 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.
- A one-fold change in pH represents a ten-fold change in [H3O+]
pOH
- The hydroxide ion molarity may be expressed as the p-function pOH.
- pOH = −log[OH−]
- Like with pH, this equation can be rearranged to calculate OH-:
- [OH−] = 10−pOH
Classifying solutions
- A solution is neutral if it contains equal concentrations of hydronium and hydroxide ions.
- This occurs at pH = 7.
- A solution acidic if it contains a greater concentration of hydronium ions than hydroxide ions.
- This occurs at pH < 7.
- A solution is basic if it contains a lesser concentration of hydronium ions than hydroxide ions.
- This occurs at pH > 7.
Test yourself on pH & pOH
What is the mathematical definition of pH?
If we rearrange the pH equation to pH = -log10[H3O+]
to make[H3O+] the subject, what will the equation be?