OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 12
1 mark · Medium difficulty · Multiple Choice
Calculate the hydrogen ion concentration resulting from mixing two strong monobasic acids HA and HB.
Practise this questionQuestion
Question text
12 HA and HB are two strong monobasic acids.
25.0 cm3 of 6.0 mol dm–3 HA is mixed with 45.0 cm3 of 3.0 mol dm–3 HB.
What is the H+(aq) concentration, in mol dm–3, in the resulting solution?
A 1.9
B 2.1
C 4.1
D 4.5
Your answer [1]
Mark scheme
Show the mark scheme
12 C 1 ALLOW 4.1 in the box
How to answer it
Question 12: Mixing Strong Acids Study Guide
This question assesses your understanding of strong acid equilibria, stoichiometry of monobasic acids, calculating moles from concentration and volume, and finding the final concentration of hydrogen ions ( H⁺(aq) ) in a combined solution. It specifically tests your ability to handle total volume changes upon mixing solutions.
Question Part 12
Calculating Hydrogen Ion Concentration in a Mixture of Strong Monobasic Acids
✅ Correct Answer
C (4.1)
The correct concentration of H⁺(aq) in the resulting solution is 4.1 mol dm⁻³ .
💡 Key Knowledge
- Strong Monobasic Acids: Fully dissociate in water. One mole of acid produces one mole of H⁺ ions (e.g., HA → H⁺ + A⁻ ).
- Moles Formula: Moles = (concentration × volume) / 1000 (when volume is in cm³).
- Total Volume: When two solutions are mixed, their volumes add together. Total Volume = Volume 1 + Volume 2.
🧠 Exam Technique
For multiple-choice calculation questions, never guess! Always write down your steps clearly in the blank space of your paper. Calculate total moles of H⁺ first, divide by the total combined volume in dm³, and check your final answer against the options.
❌ Common Errors
- Forgetting to sum volumes: Dividing total moles by only one of the initial volumes instead of the combined total volume.
- Unit conversion slips: Forgetting to divide cm³ by 1000 (or multiply by 10⁻³ ) when calculating moles, or forgetting to convert the final combined volume from cm³ back to dm³.
- Averaging concentrations: Simply adding or averaging 6.0 and 3.0 instead of accounting for different volumes and total moles.
📐 Step-by-Step Calculation
- Calculate moles of H⁺ from acid HA:
Moles = 25.0 cm³ × 6.0 mol dm⁻³ / 1000 = 0.150 mol - Calculate moles of H⁺ from acid HB:
Moles = 45.0 cm³ × 3.0 mol dm⁻³ / 1000 = 0.135 mol - Calculate total moles of H⁺:
Total moles = 0.150 + 0.135 = 0.285 mol - Calculate total volume of the mixture:
Total volume = 25.0 cm³ + 45.0 cm³ = 70.0 cm³ = 0.0700 dm³ - Calculate final [H⁺(aq)] concentration:
Concentration = Total moles / Total volume in dm³ = 0.285 / 0.0700 = 4.0714... mol dm⁻³
Rounding to 2 significant figures gives 4.1 mol dm⁻³ (matching option C).
Topics
Module 2: Foundations in chemistry · Module 5: Physical chemistry and transition elements · 5.1 Rates, equilibrium and pH · 2.1 Atoms and reactions
Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.