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S6 Chemistry Organic Chemistry (Hydrocarbons), Paper 3: combustion stoichiometry, isomers and cracking
S6 Chemistry Organic Chemistry (Hydrocarbons), Paper 3: combustion stoichiometry, isomers and cracking
Paper 3 covers the molar mass and combustion of octane, substitution of methane with chlorine, the alkane general formula, the three isomers of C4H8, finding n from an alkene molar mass, hydrogenation of propene, a combustion analysis and the cracking of decane with a mass balance.
Focus Decide the formula from the molar mass first, then convert with the coefficients of the balanced equation, and always check a cracking or addition answer against mass conservation.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S6 Chemistry: Organic Chemistry (Hydrocarbons) |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationWhat is the molar mass of octane, C8H18, and how many moles are in 2.28 g?
- FoundationOctane burns as C8H18 plus 12.5O2 giving 8CO2 and 9H2O. How many moles and grams of oxygen does 0.02 mol of octane need?
- Foundation0.10 mol of methane reacts with chlorine in ultraviolet light. How much chlorine is needed and what mass of chloromethane, molar mass 50.5, forms?
- FoundationWhat is the general formula of an alkane, and what is the molar mass of C5H12?
- IntermediateHow many structural isomers does the alkene C4H8 have, ignoring cis-trans forms, and what are they?
- Intermediate7.0 g of an alkene is exactly 0.10 mol. Find its molar mass, the value of n and its formula.
- Intermediate4.2 g of propene is hydrogenated completely to propane. How many moles and grams of hydrogen are needed?
- ChallengeBurning 0.10 mol of a hydrocarbon gives 0.30 mol of carbon dioxide and 0.40 mol of water. Find the formula, the name and the mass of 0.10 mol.
- ChallengeDecane cracks as C10H22 giving C4H10 and C6H12. From 0.50 mol of decane, find the masses of butane and hexene and check the mass balance.
- Error redoError repair: a student says alkanes decolourise bromine water because their molecules contain carbon-carbon single bonds. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 114.0 g per mole, so 0.02 mol | Eight carbons give 96.0 and eighteen hydrogens give 18.0, so the molar mass is 114.0 and 2.28 g is 0.02 mol. |
| 2 | 0.25 mol, which is 8.0 g | The ratio is one to 12.5, so 0.02 times 12.5 gives 0.25 mol, and 0.25 times 32.0 gives 8.0 g. |
| 3 | 0.10 mol of chlorine, giving 5.05 g of chloromethane | Substitution runs one to one here, so 0.10 mol of chlorine is used, and 0.10 times 50.5 gives 5.05 g of chloromethane. |
| 4 | CnH2n+2, giving 72.0 g per mole | Every carbon is saturated, so the hydrogen count is twice the carbon count plus two, giving 60.0 plus 12.0, which is 72.0. |
| 5 | Three: but-1-ene, but-2-ene and 2-methylpropene | The double bond can sit at the end or in the middle, and shortening the chain to three carbons with a methyl branch gives the third isomer. |
| 6 | 70.0 g per mole, so n equals five and the alkene is C5H10 | 7.0 over 0.10 is 70.0, and CnH2n weighs 14n, so 70 divided by 14 gives five. |
| 7 | 0.10 mol, which is 0.20 g | Propene has a molar mass of 42.0, so 4.2 g is 0.10 mol, which adds 0.10 mol of hydrogen weighing 0.20 g. |
| 8 | C3H8, propane, with a mass of 4.4 g | Carbon is three and hydrogen is eight, giving a molar mass of 44.0 and a mass of 4.4 g for 0.10 mol. |
| 9 | 29.0 g of butane and 42.0 g of hexene, which add to the 71.0 g of decane | The three molar masses are 142.0, 58.0 and 84.0, so the products weigh 29.0 g and 42.0 g, which add to the 71.0 g of decane used. |
| 10 | Wrong: alkanes are saturated and cannot add bromine, so only alkenes decolourise bromine water | Single bonds are saturated, so alkanes only undergo substitution in ultraviolet light; rapid decolourisation of bromine water needs a carbon-carbon double bond. |
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.
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