How do I find the limiting reagent?

Divide each reactant amount by its coefficient; the smaller value runs out first and limits the reaction.

Can a percentage yield exceed 100 per cent?

No; above 100 per cent means wet or impure product or a weighing error, so the experiment is repeated.

S4 Chemistry: The Mole (Paper 3)

2026-11-01

This paper covers percentage yield and limiting reagents: find the limiting reagent, then compare actual with theoretical yield.

Direct answer　Use the mole ratio to find the limiting reagent, work out the theoretical yield, then divide actual by theoretical.

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: S4 Chemistry: The Mole, Concentration and Percentage Yield
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationA reaction should give 10.0 g but gives 8.0 g. Find the percentage yield.
FoundationThe theoretical yield is 0.50 mol and 0.40 mol is obtained. What is the percentage yield?
FoundationThe theoretical yield is 25.0 g and 20.0 g is collected. What is the percentage yield?
FoundationThe theoretical yield is 24.0 g and 18.0 g is obtained. What is the percentage yield?
IntermediateHeating 10.0 g of calcium carbonate should give 4.4 g of carbon dioxide and 3.52 g is collected. Find the percentage yield.
Intermediate4.0 g of hydrogen reacts fully with 32 g of oxygen. What mass of water should form with H2 = 2, O2 = 32 and H2O = 18?
Intermediate0.30 mol of zinc reacts with 0.40 mol of hydrochloric acid. Which is limiting and how many moles of hydrogen form?
Challenge5.0 g of calcium carbonate should give 2.2 g of carbon dioxide but 1.76 g forms. Find the yield and one reason it is below 100 per cent.
ChallengeA reaction has a 75 per cent yield. What theoretical mass is needed to obtain 6.0 g of product?
Error redoError repair: a student reports a 120 per cent yield and says higher is better. Give two mistakes.

03　Answers and one-line notes

#	Answer	One-line note
1	80%	Percentage yield is actual over theoretical times one hundred, so 8.0 over 10.0 is 80 per cent.
2	80%	0.40 divided by 0.50 is 0.80, that is 80 per cent.
3	80%	20.0 divided by 25.0 is 0.80, that is 80 per cent.
4	75%	18.0 divided by 24.0 is 0.75, that is 75 per cent.
5	80%	10.0 g is 0.10 mol, so the theoretical carbon dioxide is 4.4 g and 3.52 over 4.4 is 80 per cent.
6	36 g	There are 2.0 mol of hydrogen and 1.0 mol of oxygen, exactly the two to one ratio, so 2.0 mol or 36 g of water forms.
7	the acid limits; 0.20 mol of hydrogen	0.30 mol of zinc needs 0.60 mol of acid but only 0.40 mol is present, so the acid limits and 0.20 mol of hydrogen forms.
8	80 per cent; some gas escapes or the reaction is incomplete	1.76 over 2.2 is 0.80 or 80 per cent, and escaping gas or incomplete heating lowers the actual yield.
9	8.0 g	The actual yield is 0.75 of the theoretical value, so 6.0 divided by 0.75 gives 8.0 g.
10	yield cannot exceed 100 per cent; the excess means wet or impure product	The actual yield cannot exceed the theoretical one, so above 100 per cent signals wet or impure product rather than success.

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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