Why does full substitution of methane need four times the chlorine?

Methane has four hydrogens and each replacement takes one chlorine molecule, so complete substitution takes four molecules rather than one.

Why subtract two and divide by 14 for an alkane formula?

The general formula CnH2n+2 weighs 14n plus two, so subtracting two and dividing by 14.0 gives the carbon number.

S6 Chemistry Organic Chemistry (Hydrocarbons), Paper 4: bromine addition, formula from molar mass and cracking

2026-11-02

Tutor Prep Guide · Primary English

Paper 4 covers the molar mass and combustion of heptane, the full substitution of methane to tetrachloromethane, the alkane formula from a molar mass of 128.0, the five isomers of C5H10, finding n from 11.2 g of an alkene, bromination of butene, a combustion analysis giving hexane, and the cracking of hexadecane with a mass balance.

Focus　Decide the formula from the molar mass first, then convert with the coefficients of the balanced equation; for substitution and addition count how many reagent molecules each hydrocarbon molecule takes, and finish with a mass balance.

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 heptane, C7H16, and how many moles are in 4.0 g?
FoundationHeptane burns as C7H16 plus 11O2 giving 7CO2 and 8H2O. For 0.040 mol, find the moles and mass of oxygen and the mass of carbon dioxide.
Foundation0.050 mol of methane is fully substituted in excess chlorine under ultraviolet light. How much chlorine, in moles and grams, is used, and what mass of CCl4, molar mass 154.0, forms?
FoundationGive the general formula of an alkane, then find the formula and the number of hydrogen atoms for an alkane of molar mass 128.0.
IntermediateHow many structural isomers does the alkene C5H10 have, ignoring cis-trans forms, and what are they?
Intermediate11.2 g of an alkene is exactly 0.20 mol. Find its molar mass, the value of n and the formula.
Intermediate0.10 mol of butene is fully brominated by bromine water. How many moles and grams of Br2, molar mass 160.0, are used?
ChallengeBurning 0.20 mol of a hydrocarbon gives 1.20 mol of carbon dioxide and 1.40 mol of water. Find the formula, the name and the mass of 0.20 mol.
ChallengeHexadecane cracks as C16H34 giving C8H18 and C8H16. From 0.25 mol of hexadecane, find the masses of octane and octene and check the mass balance.
Error redoError repair: a student says bromine water is decolourised because bromine replaces hydrogen atoms on the alkene. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	100.0 g per mole, so 0.040 mol	Seven carbons give 84.0 and sixteen hydrogens 16.0, so the molar mass is 100.0 and 4.0 g is 0.040 mol.
2	0.44 mol of oxygen, which is 14.08 g, giving 12.32 g of carbon dioxide	The ratio is one to eleven, so 0.44 mol of oxygen weighing 14.08 g is needed, and 0.28 mol of carbon dioxide weighs 12.32 g.
3	0.20 mol of chlorine, which is 14.2 g, giving 7.7 g of tetrachloromethane	Four hydrogens are replaced one at a time, so four molecules of chlorine per methane are needed: 0.20 mol weighing 14.2 g, and 0.050 times 154.0 gives 7.7 g of tetrachloromethane.
4	CnH2n+2, so C9H20 with twenty hydrogen atoms	The general formula weighs 14n plus two, so 128.0 minus 2 over 14 gives nine carbons and the hydrogen count is 2 times 9 plus 2, which is 20.
5	Five: pent-1-ene, pent-2-ene, 2-methylbut-1-ene, 3-methylbut-1-ene and 2-methylbut-2-ene	A straight chain of five carbons gives two alkenes, and a four-carbon chain with a methyl branch at position two or three gives three more, making five in all.
6	56.0 g per mole, so n equals four and the alkene is C4H8, butene	11.2 over 0.20 is 56.0, and CnH2n weighs 14n, so 56 divided by 14 gives four.
7	0.10 mol, which is 16.0 g of bromine	One carbon-carbon double bond adds exactly one molecule of bromine, so 0.10 mol of butene takes 0.10 mol of bromine, which weighs 16.0 g.
8	C6H14, hexane, with a mass of 17.2 g	Carbon is 1.20 over 0.20, which is six, and hydrogen is twice 1.40 over 0.20, which is fourteen, so the molar mass is 86.0 and 0.20 mol weighs 17.2 g.
9	28.5 g of octane and 28.0 g of octene, which add to the 56.5 g of hexadecane	The three molar masses are 226.0, 114.0 and 112.0, so the products weigh 28.5 g and 28.0 g, adding to the 56.5 g of hexadecane used.
10	Wrong: it is an addition reaction, with bromine atoms adding across the double bond and no hydrogen replaced	Bromine water decolourises at once in the dark at room temperature, which is the signature of addition, whereas substitution needs ultraviolet light and releases hydrogen bromide.

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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