Strength is about what fraction of the dissolved acid has ionised. Concentration is about how much acid was dissolved in the first place. A particle picture shows both on one page.
This lesson is part of acid-base reasoning beyond memorised labels in SPM Chemistry. For the standard version of the idea, see distinguishing strength from concentration.
How do I draw the two acids?
Draw ten acid units in each beaker, so the concentration is the same in both pictures.
Beaker A, a strong acid such as hydrochloric acid. Ten HCl units have become ten H⁺ and ten Cl⁻. No HCl molecules remain.
Beaker B, a weak acid such as ethanoic acid. Nine CH₃COOH molecules remain intact. One has produced one H⁺ and one CH₃COO⁻.
Count the hydrogen ions. Beaker A has ten and Beaker B has one. The concentrations are equal, but A contains far more H⁺, so A has the lower pH and the higher conductivity.
The one-in-ten ratio is illustrative. The real fraction for ethanoic acid at school concentrations is much smaller, but the picture keeps the idea clear.
Worked example: a concentrated weak acid
Question. Beaker C has 40 acid units of ethanoic acid. Beaker D has 4 units of hydrochloric acid, all ionised. Draw both and compare the number of H⁺.
Beaker C. Of 40 units, suppose 2 have ionised, leaving 38 intact molecules and giving 2 H⁺ and 2 CH₃COO⁻. Beaker D. All 4 units have ionised, giving 4 H⁺ and 4 Cl⁻.
Beaker C has ten times as much dissolved acid but only 2 H⁺, while D has 4 H⁺. D is dilute yet strong, and C is concentrated yet weak. A student who compared only the acid count would call C the stronger acid. A student who counted ions sees that D ionises completely and C does not.
What is a three-line answer template?
Use this structure whenever a question mixes strength and concentration.
- State the concentration of each solution, using the amount of solute per volume.
- State the extent of ionisation of each acid, using “completely” or “partially”.
- Link the ion count to the evidence, such as pH or conductivity.
Example: “Both solutions contain the same amount of acid per dm³. Hydrochloric acid ionises completely and ethanoic acid only partially, so hydrochloric acid has a higher hydrogen ion concentration and a lower pH.”
The mistake to avoid
The common mistake is to draw the weak acid with fewer molecules in total. A fair comparison keeps the total acid the same and changes only the ionisation. If you draw fewer molecules, you have changed the concentration and broken the test.
| Picture | What it shows |
|---|---|
| Fewer particles in total | Lower concentration |
| Same total, fewer ions | Weaker acid |
Ask yourself which variable you are changing before you draw.
Check yourself
Beaker E has 20 units of a strong acid in 1 dm³. Beaker F has 20 units of a weak acid in 1 dm³, with 3 ionised. Which has the higher pH, and what do you draw to justify it?
Answer
Beaker F has the higher pH. In E, all 20 units ionise to give 20 H⁺. In F, only 3 ionise, giving 3 H⁺ and leaving 17 intact molecules.
The total dissolved acid is the same, so the difference comes from ionisation. Fewer H⁺ means a higher pH.
What to study next
Go on to interpreting a neutralisation dataset without assuming every acid is monoprotic. You can experiment with the ideas using the concentration and dilution tutor.
If you want a teacher to ask you to draw and explain, see online one-to-one Chemistry tuition.