These eight original questions mix the four lessons. Write your answer first, then open the explanation.
If you need a lesson, return to the acid-base reasoning hub.
Questions
Question 1. Two acids have the same concentration. Acid A has pH 1 and acid B has pH 4. State which is stronger and why.
Answer
Acid A. At equal concentration, a lower pH means a higher hydrogen ion concentration, so A ionises more completely. B ionises only partially.
Question 2. Solution P is 0.50 mol dm⁻³ ethanoic acid. Solution Q is 0.0005 mol dm⁻³ hydrochloric acid. Student: “P is stronger because it is more concentrated.” Correct the statement.
Answer
P is more concentrated, not stronger. Ethanoic acid ionises partially, and hydrochloric acid ionises completely, so Q is the stronger acid. See distinguishing strength from concentration.
Question 3. Draw ten units of weak acid in water with two ionised. How many H⁺ and how many intact molecules are shown?
Answer
Two H⁺ with two anions, and eight intact molecules. The picture shows partial ionisation with the total acid kept at ten units.
Question 4. 25.0 cm³ of 0.100 mol dm⁻³ NaOH needs 10.0 cm³ of acid of concentration 0.125 mol dm⁻³. Is the acid monoprotic or diprotic?
Answer
Moles of NaOH = 0.00250 mol. Moles of acid = 0.0100 × 0.125 = 0.00125 mol.
Ratio = 0.00250 ÷ 0.00125 = 2, so the acid is diprotic.
Question 5. 40.0 cm³ of 0.300 mol dm⁻³ hydrochloric acid is diluted to 120 cm³. Find the new concentration and the moles of acid after dilution.
Answer
Moles = 0.0400 × 0.300 = 0.0120 mol, unchanged by dilution. New concentration = 0.0120 ÷ 0.120 = 0.100 mol dm⁻³.
Question 6. A student says, “Diluting a strong acid makes it a weak acid.” Write a two-sentence correction.
Answer
Dilution reduces the concentration of the acid, not its strength. A strong acid still ionises completely after dilution, but its hydrogen ion concentration is lower and its pH is higher.
Question 7. Using the model “water ionises slightly into equal H⁺ and OH⁻”, decide whether this is consistent: A solution with pH 9 contains hydroxide ions but no hydrogen ions.
Answer
Inconsistent. In the model, pH 9 means OH⁻ exceeds H⁺, but some H⁺ remains because water ionises. The statement should say “very few hydrogen ions”. See reconciling a pH description with an explicitly stated classroom model.
Question 8. 20.0 cm³ of 0.250 mol dm⁻³ sulfuric acid is diluted to 100 cm³, and 25.0 cm³ of the diluted solution is neutralised by NaOH. Find the volume of 0.100 mol dm⁻³ NaOH needed.
Answer
Moles of H₂SO₄ in the original = 0.0200 × 0.250 = 0.00500 mol. Diluted concentration = 0.00500 ÷ 0.100 = 0.0500 mol dm⁻³.
In 25.0 cm³: moles = 0.0250 × 0.0500 = 0.00125 mol. The equation H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O gives 0.00250 mol NaOH. Volume = 0.00250 ÷ 0.100 = 0.0250 dm³, which is 25.0 cm³.
If you got these wrong
Match each question to a lesson: questions 1 to 3 to separating concentration from strength using particle models, question 4 to the neutralisation dataset lesson, questions 5 and 6 to the dilution lesson, and question 7 to the pH model lesson. Question 8 combines dilution and titration.
Log the idea you missed with the mistake log and paper-error review tool. For a teacher to review your working with you, see online one-to-one Chemistry tuition.