H⁺ Ions and pH

AQA-style Exam Question

A solution of a strong acid with pH 2.0 has a concentration of H⁺ ions of 1.0 × 10⁻² mol/dm³ .

What is the concentration of H⁺ ions in a solution of the same acid with pH 3.0?

[1 mark]

• What is the relationship between H⁺ ions and acidity or pH?

  Increase in concentration of H⁺ ions means:

  • Increase in acidity

  • Decrease in pH value

• What's the rule for H⁺ ions and pH?

  If pH drops by 1, H⁺ concentration multiplies by 10:

  • pH -1 means H⁺ x 10

  • pH -2 means H⁺ x 100

  • pH - 3 means H⁺ x 1000

  Also:

  • pH +1 means H⁺ ÷ 10

  • pH +2 means H⁺ ÷ 100

  • pH +3 means H⁺ ÷ 1000

• According to the question, what happened to the pH?

  It went from 2 to 3 so:

  +1

• So what will happen to H⁺ ions?

  ÷ 10 so:

  1 × 10⁻² ÷ 10

  = 1 × 10⁻³ mol/dm³

  (1 mark)

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