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S6 Chemistry: Polymers and Plastics (Paper 3: Challenge)
S6 Chemistry: Polymers and Plastics (Paper 3: Challenge)
Ten items on mass percentages, degrees of polymerisation, limiting reagents and oxygen demand, all from molar masses.
Direct answers Answers in order: 192, 36, 76.0 per cent, 2 000, 85.7 per cent, 2.0 x 10^-3 mol, 36 g, hydrogen chloride, 1.07 mol, the two water molecules were not subtracted.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S6 Chemistry: Polymers and Plastics |
| Level | S6 |
02 Questions (10 items)
- FoundationWhat is the relative mass of the PET repeat unit C10H8O4? (C 12, H 1, O 16)
- FoundationEthane-1,2-diol (Mr 62) and benzene-1,4-dicarboxylic acid (Mr 166) form one PET repeat unit. What mass of water is lost?
- FoundationWhat percentage by mass of the PTFE repeat unit C2F4 is fluorine? (F 19)
- IntermediateA PVC sample has relative molecular mass 125 000. What is its degree of polymerisation? (Repeat unit 62.5)
- IntermediateWhat percentage by mass of the poly(propene) repeat unit C3H6 is carbon?
- IntermediateA 45.2 g nylon sample has relative molecular mass 22 600. How many moles of polymer molecules is that?
- Challenge2.0 mol of hexanediamine condenses with excess hexanedioic acid. What mass of water is lost at most?
- ChallengeCompared with poly(ethene), burning PVC gives one extra choking gas. Which one?
- Challenge10.0 g of poly(ethene) burns completely. How many moles of oxygen are needed? (Each mole of C2H4 needs 3 mol O2)
- Error redoError replay: a student adds 62 and 166 to get 228 for the PET repeat unit. What went wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 192 | Adding 120, 8 and 64 gives 192, which is 36 less than the two monomers added together. |
| 2 | 36 | Two ester links each lose one water, and 62 plus 166 minus 36 is the 192 repeat unit. |
| 3 | 76.0% | Four fluorines weigh 76 and the repeat unit weighs 100, so the percentage is exactly 76. |
| 4 | 2 000 | Divide 125 000 by 62.5; shifting the decimal point twice gives 2 000. |
| 5 | 85.7% | Carbon accounts for 36 out of 42, and 36 over 42 is 0.857. |
| 6 | 2.0 x 10^-3 mol | Moles are mass over relative molecular mass, and 45.2 over 22 600 is 0.0020. |
| 7 | 36 g | The diacid is in excess, so the diamine limits it: 2.0 mol of repeat units means 2.0 mol of water. |
| 8 | Hydrogen chloride (HCl) | The chlorine in PVC becomes hydrogen chloride on burning and forms an acid in water, so PVC flue gas needs scrubbing. |
| 9 | 1.07 mol | Divide 10 by 28 to get 0.357 mol, then multiply by three to obtain 1.07 mol. |
| 10 | Two water molecules were not subtracted, so it should be 192 | Each repeat unit has two ester links and each link loses one water, so 36 must be taken away. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
Before multiplying by the degree of polymerisation, subtract the small molecules that left.Tutor Registration →
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