What does the pH scale measure?

The pH scale measures the concentration of hydrogen ions (H+) in a solution.

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What pH values are associated with acids and alkalis?

  • Acids have pH values below 7.
  • Alkalis have pH values above 7.


A pH of 7 is considered neutral, neither acidic nor alkaline.

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What is the formula to calculate the pH of a solution from the concentration of H+ ions?

pH = -log10[H+(aq)]

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What is the formula to calculate the concentration of H+ ions in a solution from its pH?

[H+(aq)] = 10-pH

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How does the concentration of H+ ions change as the pH of a solution decreases?

As the pH of a solution decreases by one unit, the concentration of H+ ions increases by a factor of 10.


For example, if pH decreases from 7 to 6, [H+] increases from 10-7 to 10-6 mol dm-3.

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What are the steps involved in calculating the pH of a strong base from the concentration of OH- ions in solution?

The steps to calculate the pH of a strong base from [OH-] are:

  1. Rearrange the ionic product of water to find [H+]:

[H+]=[OH−]Kw​​

  1. Use [H+] to find pH:

pH = -log10[H+]

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In what way do the relative concentrations of H+ and OH- ions determine whether a solution is acidic, neutral, or alkaline?

  • Acidic solution: [H+] > [OH-].
  • Neutral solution: [H+] = [OH-].
  • Alkaline solution: [H+] < [OH-].

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Why does water have a neutral pH?

Water has a neutral pH of 7 because it each molecule ionises to produce equal concentrations of H+ and OH- ions. This is shown by the equation:

H2O(l) ⇌ H+(aq) + OH-(aq).

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