How is paper 4 different from the first three?

Paper 4 drills the commonest titration slips: mole ratios, unit conversion, dilution and back titration, with shorter items but every trap present.

Why is half the volume enough for sulfuric acid?

Each sulfuric acid molecule supplies two protons, so only half as many moles are needed to neutralise the same base.

What is the order of steps in a back titration?

Add up the total acid added, subtract the excess measured by the base, and the difference is what reacted with the sample.

What should I do after a wrong answer?

Write down whether the slip was the mole ratio, the units or the dilution, then redo a similar item.

S5 Chemistry acid-base titration paper 4: error drills and common slips

2026-11-10

Tutor Prep Guide · S5 Chemistry Acid-Base Titration

Ten items target mole ratios, mass concentration, dilution, purity and back titration, each with a one-line note.

Direct answers　Items 01 to 03 cover basic neutralising volumes and moles, 04 to 06 titration to find concentration, mass concentration and dilution, 07 to 09 purity, back titration and diprotic acids, and 10 the mole-ratio slip.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S5 Chemistry Acid-Base Titration
Level	S5

02　Questions (10 items)

FoundationHow much 0.100 M hydrochloric acid neutralises 25.0 cm3 of 0.100 M sodium hydroxide?
FoundationHow much 0.100 M sodium hydroxide neutralises 20.0 cm3 of 0.150 M hydrochloric acid?
FoundationHow many moles of solute are in 250 cm3 of a 0.040 M solution?
Intermediate0.120 M sodium hydroxide titrates 25.0 cm3 of hydrochloric acid, average titre 22.50 cm3. Find the acid concentration.
IntermediateConvert 0.108 M hydrochloric acid (relative molecular mass 36.5) into g/dm3.
Intermediate25.0 cm3 of 2.00 M hydrochloric acid is diluted to 500 cm3. Find the new concentration.
Challenge1.50 g of impure sodium hydroxide is made up to 250 cm3. A 25.0 cm3 portion needs 24.00 cm3 of 0.120 M hydrochloric acid. Find the percentage purity.
Challenge1.00 g of impure calcium carbonate reacts with 50.0 cm3 of 0.500 M hydrochloric acid; the excess acid needs 22.0 cm3 of 0.250 M sodium hydroxide. Find the purity.
Challenge20.0 cm3 of sulfuric acid needs 30.0 cm3 of 0.200 M sodium hydroxide. Find its concentration and convert it to g/dm3 (relative molecular mass 98.0).
Error redoError drill: a student writes “25.0 cm3 of 0.100 M sodium hydroxide needs 25.0 cm3 of 0.100 M sulfuric acid because the concentrations match”. Point out the mistake and give the correct volume.

03　Answers and one-line notes

#	Answer	One-line note
1	Moles = 0.100 x 0.0250 = 2.50 x 10^-3 mol, so volume = 25.0 cm3	The ratio is 1:1, so equal concentrations need equal volumes.
2	Moles = 0.150 x 0.0200 = 3.00 x 10^-3 mol, volume = 3.00 x 10^-3 / 0.100 = 30.0 cm3	Find the moles of acid first, then divide by the base concentration.
3	Moles = 0.040 x 0.250 = 0.0100 mol	Convert the volume from cm3 to dm3 before multiplying.
4	Moles = 0.120 x 0.02250 = 2.70 x 10^-3 mol, so concentration = 2.70 x 10^-3 / 0.0250 = 0.108 M	Work out the moles of base, then use the 1:1 ratio for the acid.
5	0.108 x 36.5 = 3.94 g/dm3	Mass concentration is molarity multiplied by relative molecular mass.
6	0.100 M	The moles do not change on dilution, so c1V1 = c2V2.
7	Moles of base = 2.88 x 10^-3 mol in the portion, so 2.88 x 10^-2 mol in 250 cm3, mass 1.152 g; purity = 76.8%	Scale the moles up to the whole flask before converting to mass.
8	Total acid 0.0250 mol, excess 5.50 x 10^-3 mol, so 0.0195 mol reacted; that is 9.75 x 10^-3 mol of carbonate, 0.975 g, purity 97.5%	In a back titration subtract the excess acid from the total acid first.
9	Base moles = 6.00 x 10^-3, acid moles = 3.00 x 10^-3, so 0.150 M; mass concentration = 0.150 x 98.0 = 14.7 g/dm3	Sulfuric acid has two ionisable protons, so the ratio is 1:2.
10	The student ignores that sulfuric acid is diprotic; base moles = 2.50 x 10^-3, so only 1.25 x 10^-3 mol of acid is needed, volume = 12.5 cm3	Equal concentrations do not mean equal moles; count the protons each molecule supplies.

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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