OCR A-Level Chemistry AS Breadth in chemistry (01), June 2024: Question 8
1 mark · Medium difficulty · Multiple Choice
Identify the correct statement about the treatment of drinking water with chlorine regarding toxic chlorinated hydrocarbons.
Practise this questionQuestion
Question text
8 In the UK, water companies typically treat drinking water with chlorine gas at a concentration of
0.500 mg dm–3 or less.
Which statement about UK drinking water is correct?
A Chlorine in drinking water can catalyse the breakdown of ozone.
B Chlorine may form toxic chlorinated hydrocarbons.
C Drinking water with a chlorine gas concentration of 0.500 mg dm–3 contains 2.12 × 1018
chlorine molecules in each dm3.
D In hot weather, chlorine can vaporise from drinking water to cause global warming.
Your answer [1]
Mark scheme
Show the mark scheme
8 B 1
How to answer it
Water Treatment and Chlorine Chemistry
What this question tests
This question assesses your understanding of Group 7 (halogens) chemistry, specifically the benefits and risks of using chlorine in water treatment, coupled with the ability to perform quantitative mole calculations involving concentrations and Avogadro's constant.
Exam Breakdown
✅ Correct Answer: B
Chlorine reacts with trace organic matter naturally present in water sources (such as decomposition products from vegetation) to form toxic chlorinated hydrocarbons (e.g., chloromethanes/trihalomethanes).
💡 Key Knowledge
Chlorine is added to drinking water to kill harmful bacteria and sterilise the supply. However, safety tradeoffs exist regarding toxic by-products, which is why monitoring and keeping concentrations low (to 0.500 mg dm⁻³ ) is critical.
🧠 Exam Technique
When faced with a multiple-choice question combining chemical facts and calculations (like option C), evaluate the straightforward chemical statements first. Option B is a well-known, direct fact from the OCR specification regarding water sterilisation risks.
❌ Common Errors
Students often get distracted by complex-looking calculation options like C without checking the maths, or confuse chlorine's role in water treatment with ozone depletion in the upper atmosphere (which is caused by CFCs, not dissolved water treatment chlorine).
Why Option C is Incorrect (Calculation Walkthrough)
Let's verify why statement C is mathematically false to understand why B is the only valid choice:
📐 Step-by-Step Verification for Option C
- Find concentration in g dm⁻³: Convert 0.500 mg dm⁻³ to grams by dividing by 1000.
0.500 mg dm⁻³ = 0.000500 g dm⁻³ (or 5.00 × 10⁻⁴ g dm⁻³ ). - Calculate moles of Cl₂ per dm⁻³: Divide mass by the molar mass of Cl₂ ( 71.0 g mol⁻³ ).
Moles = (5.00 × 10⁻⁴) / 71.0 = 7.04 × 10⁻⁶ mol dm⁻³ . - Calculate number of molecules: Multiply by Avogadro's constant ( 6.022 × 10²³ ).
Molecules = 7.04 × 10⁻⁶ × 6.022 × 10²³ = 4.24 × 10¹⁸ molecules dm⁻³ . - Conclusion: The calculated value ( 4.24 × 10¹⁸ ) does not match the 2.12 × 10¹⁸ stated in option C, proving option C is numerically incorrect.
Topics
Module 3: Periodic table and energy · 3.1 The periodic table
Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.