Why concentrated rather than dilute sulphuric acid?

The concentrated acid both supplies protons and removes the water formed, which pulls the equilibrium right, while a dilute acid adds water and pushes it back.

Why check a hydrolysis with mass balance?

Only one water molecule is added, so the mass of the ester plus the water must equal the mass of the alcohol plus the acid.

S6 Chemistry Organic Chemistry (Functional Groups), Paper 2: oxidation, esterification and hydrolysis

2026-11-02

Tutor Prep Guide · Primary English

Paper 2 works through ethanol with sodium, ethanoic acid with carbonate, limiting reagents in esterification, dehydration to ethene, oxidation demand, the isomers of C3H6O2, ester hydrolysis with a mass balance and an alcohol combustion analysis.

Focus　Write the balanced equation first, then take the mole ratio; esterification and hydrolysis both run one to one, so the limiting reagent is decided by moles rather than mass.

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 (Functional Groups)
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationWhat is the molar mass of ethanol, C2H5OH, and how many moles are in 2.3 g?
Foundation0.10 mol of ethanol reacts with excess sodium, two moles giving one mole of hydrogen. How many moles and grams of hydrogen form?
Foundation0.10 mol of ethanoic acid reacts with excess sodium carbonate. What volume of carbon dioxide forms at 24 dm3 per mole?
Foundation0.20 mol of ethanol and 0.15 mol of ethanoic acid are heated with concentrated sulphuric acid. Which is limiting, and what is the maximum mass of ethyl ethanoate?
Intermediate0.10 mol of ethanol is dehydrated at 170 degrees Celsius. What amount, mass and volume of ethene results at 24 dm3 per mole?
Intermediate0.10 mol of ethanol is oxidised completely to ethanoic acid. How many moles and grams of oxygen are needed?
IntermediateThe formula C3H6O2 fits both a carboxylic acid and an ester. Give one example of each and state how many such isomers exist.
Challenge8.8 g of ethyl ethanoate is hydrolysed completely. Find the moles and masses of ethanol and ethanoic acid formed.
ChallengeBurning 0.10 mol of a saturated alcohol gives 0.20 mol of carbon dioxide and 0.30 mol of water. Find its formula, its name and the mass of 0.10 mol.
Error redoError repair: a student claims that mixing ethanol and ethanoic acid simply makes ethyl ethanoate. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	46.0 g per mole, so 0.05 mol	Two carbons give 24.0, six hydrogens 6.0 and one oxygen 16.0, so 46.0 g per mole, and 2.3 g is 0.05 mol.
2	0.05 mol, which is 0.10 g	Two moles of ethanol release one mole of hydrogen, so half of 0.10 mol is 0.05 mol and its mass is 0.10 g.
3	0.05 mol, which is 1.2 dm3	Two moles of the acid release one mole of carbon dioxide, so 0.05 mol is produced and 0.05 times 24 is 1.2 dm3.
4	The acid is limiting, giving 0.15 mol or 13.2 g of ester	The alcohol and the acid react one to one and the acid runs out first, so 0.15 times 88.0 gives 13.2 g.
5	0.10 mol, which is 2.8 g and 2.4 dm3	Each mole of ethanol loses one water molecule and gives one mole of ethene, so 0.10 times 28.0 is 2.8 g and its volume is 2.4 dm3.
6	0.10 mol, which is 3.2 g	The equation balances with one oxygen molecule per ethanol, so 0.10 mol of oxygen weighs 3.2 g.
7	Propanoic acid and methyl ethanoate, two isomers in all	A carboxylic acid needs one COOH group and the remaining two carbons give propanoic acid, while an ester needs one COO link and gives methyl ethanoate, so there are two.
8	0.10 mol of each, so 4.6 g of ethanol and 6.0 g of acid	8.8 over 88.0 is 0.10 mol and the hydrolysis is one to one, so the masses are 4.6 g and 6.0 g, which add to 10.6 g, the same as 8.8 g plus 1.8 g of water.
9	C2H6O, ethanol, with a mass of 4.6 g	Carbon is two and hydrogen is six, a saturated alcohol adds one oxygen, and 0.10 mol of the 46.0 molar mass weighs 4.6 g.
10	Wrong: esterification needs concentrated sulphuric acid and heat, and it is reversible so the yield stays below 100 per cent	Without a catalyst and heat the reaction is far too slow, and because it is reversible the water formed pushes the equilibrium back, so the yield never reaches 100 per cent.

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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