What is a back titration?

An excess of reagent is added and the remainder is titrated, which suits substances that cannot be titrated directly.

Why is the equivalence pH 7 for a strong acid and strong base?

The salt formed does not hydrolyse, so the ions present only come from water itself.

How does the degree of ionisation of a weak acid affect the curve?

A small degree of ionisation gives a higher starting pH and a shorter jump near the equivalence point.

How can titration errors be reduced?

Use one burette, read at eye level and add the last drops slowly while swirling.

S5 Chemistry titration paper 3: back titration and weak acid calculations

2026-11-10

Tutor Prep Guide · S5 Chemistry Acid-Base Titration

Ten items cover back titration, a carbonate and hydrogencarbonate mixture, weak acid ionisation, titration error and relative molecular mass.

Direct answers　Items 01 to 02 cover back titration and mixture analysis, 03 to 05 weak acid ionisation and indicators, 06 to 09 concentration, titration error and molar mass, and 10 subtracting volumes instead of moles.

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)

FoundationA 2.00 g sample containing calcium carbonate is treated with 50.0 cm3 of 0.500 M hydrochloric acid, and the excess acid needs 20.0 cm3 of 0.250 M sodium hydroxide. Find the mass fraction of calcium carbonate.
IntermediateA 25.0 cm3 solution of sodium carbonate and sodium hydrogencarbonate needs 12.0 cm3 of 0.100 M acid to the phenolphthalein end point, then a further 30.0 cm3 to the methyl orange end point. Find the carbonate concentration.
IntermediateIn the same titration, what is the concentration of the sodium hydrogencarbonate?
ChallengeA 0.100 M weak acid HA is 1.3 percent ionised. Find the pH to two decimal places.
ChallengeFor a weak acid titrated with sodium hydroxide, is the equivalence pH above, equal to or below 7, and which indicator should be used?
FoundationDissolve 4.90 g of sulphuric acid, molar mass 98.0, to make 250 cm3. Find the molarity.
ChallengeA burette reads 0.50 cm3 at the start and 24.60 cm3 at the end. If a student forgets to subtract the initial reading, what is the percentage error in the volume?
FoundationWhat volume of 0.150 M sodium hydroxide fully neutralises 15.0 cm3 of 0.200 M phosphoric acid?
Intermediate1.20 g of a monobasic acid is made up to 250 cm3, and 25.0 cm3 of it needs 24.00 cm3 of 0.100 M sodium hydroxide. Find its relative molecular mass.
Error redoA student subtracts 20.0 cm3 from 50.0 cm3 and treats 30.0 cm3 as the acid that reacted with calcium carbonate. What is the correct number of moles of acid used?

03　Answers and one-line notes

#	Answer	One-line note
1	50.0 percent	Of the 0.0250 mol of acid, 0.00500 mol is left over, so 0.0200 mol reacts, giving 0.0100 mol or 1.00 g of calcium carbonate.
2	0.0480 M	To the phenolphthalein end point only carbonate becomes hydrogencarbonate, using 0.00120 mol over 0.0250 dm3.
3	0.0720 M	Of the 30.0 cm3 in the second stage, 12.0 cm3 finishes the carbonate and the remaining 18.0 cm3 belongs to the original hydrogencarbonate.
4	2.89	The hydrogen ion concentration is 0.100 times 0.013, and the negative logarithm is taken.
5	Above 7, so use phenolphthalein	The salt of a weak acid is basic at the equivalence point, and phenolphthalein changes in the alkaline range.
6	0.200 M	4.90 g is 0.0500 mol, divided by 0.250 dm3.
7	About 2.07 percent	The true volume is 24.10 cm3, so 0.50 cm3 too much is 2.07 percent of 24.10 cm3.
8	60.0 cm3	Phosphoric acid supplies three hydrogen ions, 0.00900 mol in all, divided by 0.150 M.
9	50.0	The whole 250 cm3 holds 0.0240 mol, so 1.20 g is divided by 0.0240 mol.
10	0.0200 mol	The concentrations differ, so each volume must first become moles, then 0.0250 minus 0.00500.

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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