Learning focus
Develop precise chemical language, connect equations and practical observations, interpret periodic trends and apply salt-preparation rules to unfamiliar examples.
Two independent variables
Strength describes the proportion of acid molecules that dissociate. Concentration describes the amount of acid dissolved per unit volume. A strong acid can be dilute, and a weak acid can be concentrated.
pH depends on H+ concentration
The pH observed depends on the final hydrogen ion concentration, which is influenced by both strength and analytical concentration. Therefore, a sufficiently concentrated weak acid may have a lower pH than a very dilute strong acid.
Fair comparisons
To investigate strength, compare acids at equal molar concentration and temperature. To investigate concentration, use the same acid at different concentrations. Mixing these variables gives an ambiguous conclusion.
Exam data
Questions may provide pH, conductivity or reaction-rate data. Use the controlled conditions to decide whether the evidence concerns strength or concentration. Do not infer strength from acid volume alone.
Practical or data skill
Design a matrix comparing strong/weak and concentrated/dilute descriptions. Predict relative pH qualitatively, then note that actual values require data.
Examination tip
Never use strong and concentrated as synonyms.
Review questions and suggested answers
Question 1
Can a weak acid be concentrated?
Suggested answer
Yes.
Question 2
What must be controlled when comparing acid strength by reaction rate?
Suggested answer
Concentration, temperature, volume and reactant conditions.
Question 3
What directly determines pH?
Suggested answer
Hydrogen ion concentration in the solution.
