All subjects · Chemistry · WCH14 (Unit 4)

Brønsted-Lowry acids and the pH scale

At A2, acids and bases are defined by proton transfer, and acidity is measured on the logarithmic pH scale. This lesson covers strong acids; weak acids follow next.

Brønsted-Lowry theory (spec 14.1, 14.2)

An acid is a proton (H+) donor; a base is a proton acceptor.
Every acid-base reaction transfers a proton, making two conjugate pairs that differ by one H+:

HCl + H2O -> Cl- + H3O+ (HCl/Cl- and H3O+/H2O)

NH3 + H2O ⇌ NH4+ + OH- (NH4+/NH3 and H2O/OH-)

Water can act as an acid or a base.

pH (spec 14.3 to 14.5)

pH = -log10[H+] and [H+] = 10^-pH.

Each pH unit is a factor of 10 in [H+]: pH 2 has ten times the [H+] of pH 3. Give pH to 2 decimal places.

Strong acids (spec 14.6, 14.7)

A strong acid dissociates fully in water: HCl -> H+ + Cl-. A weak acid dissociates only partly: CH3COOH ⇌ CH3COO- + H+. Strength (degree of dissociation) is not the same as concentration.

For a strong monoprotic acid, [H+] = concentration of the acid, so 0.0100 mol dm^-3 HCl has pH 2.00. Diluting a strong acid ten times raises its pH by 1.

Worked example

Find the pH of 0.0250 mol dm^-3 hydrochloric acid, and [H+] in a solution of pH 3.40.

HCl is strong: [H+] = 0.0250 mol dm^-3
pH = -log10(0.0250) = 1.60
[H+] = 10^-3.40

Answer: pH = 1.60; [H+] = 3.98 x 10^-4 mol dm^-3

Key idea: Acid = proton donor, base = proton acceptor; conjugate pairs differ by H+. pH = -log10[H+]. Strong acids dissociate fully, so [H+] = [acid] for HCl.

Check yourself

What is the conjugate base of H2SO4?
- SO4²-
- H3SO4+
- HSO4-
- H2O
Show answer
HSO4-. Remove one H+ from H2SO4.
In the forward reaction of NH3 + H2O ⇌ NH4+ + OH-, which species acts as a Brønsted-Lowry acid?
- H2O
- NH3
- OH-
- Neither reactant, as no proton moves
Show answer
H2O. Water donates a proton to ammonia in the forward reaction.
Calculate the pH of 0.0400 mol dm^-3 hydrochloric acid.
Show answer
1.40. pH = -log10(0.0400) = 1.40.
A solution has pH 3.40. Calculate [H+].
Show answer
0.000398 mol dm^-3. [H+] = 10^-3.40 = 3.98 x 10^-4 mol dm^-3.
What is meant by a strong acid?
- A concentrated acid
- An acid with a high pH
- An acid that reacts with metals
- An acid that dissociates fully in water
Show answer
An acid that dissociates fully in water. Strength describes the degree of dissociation, not concentration.
A strong acid of pH 1.50 is diluted by a factor of 100. What is the new pH?
Show answer
3.50. [H+] falls by 100 = 10², so pH rises by 2 to 3.50.
Which pair is a conjugate acid-base pair?
- HCl and OH-
- H3O+ and OH-
- H2SO4 and SO4²-
- NH4+ and NH3
Show answer
NH4+ and NH3. NH4+ and NH3 differ by exactly one proton.

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Last updated 7 November 2026. Spotted a mistake? Tell us.