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Chemistry · Grades 9–12

Acids, bases and logarithmic pH

State the model, temperature and difference between strength and concentration.

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Learn the idea

Brønsted–Lowry

An acid donates a proton and a base accepts one.

pH

For dilute idealized aqueous school problems, pH ≈ −log₁₀[H₃O⁺], with concentration in mol/L relative to the standard state.

Strength

Strong describes extent of ionization under the stated conditions; concentrated describes amount per volume.

Water

At 25 °C in the familiar dilute model, pH + pOH ≈ 14. Neutral pH changes with temperature.

Worked examples

A Brønsted–Lowry acid does what?

The definition concerns proton transfer.

Donates a proton.

Using the dilute school model, find pH when [H₃O⁺] = 10⁻³ mol/L.

pH = −log₁₀(10⁻³) = 3.

3

Check your understanding

  1. At 25 °C with pH + pOH = 14, find pOH for pH 5.

  2. Does strong acid mean highly concentrated acid?

    1. Only for every solution below pH 7.
    2. Yes, always.
    3. No; strength and concentration are different quantities.

  3. A solution has pH 2 rather than pH 4 in the same dilute model. How many times greater is its hydronium concentration?

Answers and marking notes

Answers and marking notes
  1. 9

    14 − 5 = 9.

  2. No; strength and concentration are different quantities.

    Extent of ionization is not amount per volume.

  3. 100

    A decrease of two pH units means a factor of 10².

Further reading and review references

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Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.