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S6 Chemistry: Alkanes and Alkenes (Paper 3: challenge)
S6 Chemistry: Alkanes and Alkenes (Paper 3: challenge)
Ten challenge items on ethene from cracking decane, the molar mass of poly(ethene), a volume drop on addition, an alkene found from carbon dioxide, air for butane, the yield of dibromoethane, the case of C6H6, carbon and hydrogen counts from combustion, an alkane from a mass of carbon dioxide, and the definition of isomers.
Direct answers Answers in order: about 197.2 g, 42 000, 40.0 percent, C3H6 with molar mass 42, about 61.9 dm3, 188 g, C6H6 which is neither, C4H10, C3H8, and the same molecular formula with a different structure.
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: Alkanes and Alkenes |
| Level | S6 |
02 Questions (10 items)
- FoundationDecane, C10H22, cracks into octane and ethene. What mass of ethene can be obtained in theory from 1000 g of decane?
- FoundationA poly(ethene) molecule contains 1500 repeat units. What is its molar mass?
- FoundationAfter 10 cm3 of a mixture of ethene and hydrogen reacts completely by addition, the volume falls by 4 cm3. Find the percentage by volume of ethene in the mixture.
- Intermediate0.10 mol of an alkene gives 0.30 mol of carbon dioxide on complete combustion. Find its molecular formula and molar mass.
- IntermediateWhat volume of air is needed to burn 2 dm3 of butane completely, taking air as 21 percent oxygen?
- Intermediate28 g of ethene reacts completely with excess bromine. What mass of 1,2-dibromoethane is obtained?
- ChallengeA hydrocarbon contains 92.3 percent carbon and 7.7 percent hydrogen with a relative molecular mass of 78. Find the molecular formula and state whether it is an alkane or an alkene.
- Challenge0.50 mol of an alkane gives 2.5 mol of water and 2.0 mol of carbon dioxide on complete combustion. Find its molecular formula.
- Challenge4.4 g of an alkane burns completely to give 13.2 g of carbon dioxide. Find its molecular formula.
- Error redoA student writes that structural isomers have different molecular formulas and therefore different properties. What is correct?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | About 197.2 g | 1000 divided by 142 gives 7.0423 mol and each mole of decane gives one mole of ethene, so multiplying by 28 gives about 197.2 g. |
| 2 | 42 000 | Each repeat unit C2H4 has a molar mass of 28 and 28 times 1500 gives 42 000. |
| 3 | 40.0 percent | Ethene and hydrogen add in a 1 to 1 ratio so the fall in volume equals the volume of ethene, and 4 divided by 10 times 100 gives 40.0 percent. |
| 4 | C3H6 with molar mass 42 | The number of carbon atoms is 0.30 divided by 0.10 which is 3, and an alkene has twice as many hydrogen atoms, so the formula is C3H6. |
| 5 | About 61.9 dm3 | 2 dm3 of butane needs 13 dm3 of oxygen and 13 divided by 0.21 gives about 61.9 dm3 of air. |
| 6 | 188 g | 28 g of ethene is one mole, which adds one mole of bromine to give one mole of product with molar mass 28 plus 160, which is 188. |
| 7 | C6H6, which is neither an alkane nor an alkene | The empirical formula is CH and 78 divided by 13 gives 6 so the formula is C6H6, whose hydrogen count fits neither CnH2n+2 nor CnH2n. |
| 8 | C4H10 | The number of carbon atoms is 2.0 divided by 0.5 which is 4 and the water is 2.5 divided by 0.5 which is 5 mol, giving 10 hydrogen atoms, so the formula is C4H10. |
| 9 | C3H8 | 4.4 divided by 44 gives 0.10 mol and 13.2 divided by 44 gives 0.30 mol of carbon dioxide, so there are three carbon atoms and the alkane is C3H8. |
| 10 | Structural isomers have the same molecular formula but a different arrangement of atoms, so their properties differ | Isomerism means the same molecular formula with a different structure, as in butane and 2-methylpropane. |
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.
Write and balance the equation first, then convert with the mole ratio to mass or volume.Tutor Registration →
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