Why is the equivalence point above pH 7?

Only the salt of the weak acid remains, and its anion hydrolyses to give hydroxide ions.

What comes first in a back titration?

Work out the total moles of acid added, then subtract the acid equivalent of the base used in the back titration.

S5 Chemistry Titration and pH, Paper 2: titration curves, indicators and back titration

2026-11-02

Ten graded items from diprotic volumes through weak acid curves and indicator choice to a back titration, each with an answer.

Focus　A weak acid titrated by a strong base ends above pH 7, so use phenolphthalein; subtract the excess acid in a back titration; water in the flask changes nothing.

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: S5 Chemistry: Acid-Base Titration and pH
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationWhat volume of 0.050 mol dm^-3 sulphuric acid neutralises 25.0 cm^3 of 0.100 mol dm^-3 sodium hydroxide?
FoundationWhat is the pH of 0.0010 mol dm^-3 hydrochloric acid?
FoundationWhat is the pH of 0.0010 mol dm^-3 sodium hydroxide at 25 C?
FoundationIn a strong acid-strong base titration the pH jump is very wide. Which statement is correct?
IntermediateWhy is the pH at the equivalence point above 7 when 0.100 mol dm^-3 sodium hydroxide titrates a weak acid HA?
IntermediateWhich indicator suits the titration of a weak acid with a strong base?
IntermediateWhat is the pH of 0.050 mol dm^-3 sulphuric acid, fully ionised?
Challenge0.500 g of a sample containing calcium carbonate is treated with 50.0 cm^3 of 0.200 mol dm^-3 hydrochloric acid, and the excess needs 15.0 cm^3 of 0.100 mol dm^-3 sodium hydroxide. Find the percentage by mass of calcium carbonate.
Challenge20.0 cm^3 of 0.100 mol dm^-3 hydrochloric acid is mixed with 30.0 cm^3 of 0.100 mol dm^-3 sodium hydroxide. What is the pH?
Error redoError repair: a student says adding 20 cm^3 of distilled water to the conical flask increases the acid needed. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	25.0 cm^3	There is 0.00250 mol of alkali; the diprotic acid needs 0.00125 mol, which divides by 0.050 to give 25.0 cm^3.
2	pH 3.00	The acid ionises fully, so the hydrogen ion concentration is 0.0010 and the pH is 3.00.
3	pH 11.00	The hydroxide concentration is 0.0010, so pOH is 3.00 and the pH is 14 minus 3.00.
4	Both phenolphthalein and methyl orange are suitable	For a strong acid and strong base the equivalence point is at pH 7, and both indicators change colour inside the sharp jump.
5	The salt of a weak acid hydrolyses to release hydroxide ions	At the equivalence point only the salt of a weak acid remains, and its anion hydrolyses to give hydroxide ions.
6	Phenolphthalein	The equivalence point is weakly alkaline and phenolphthalein changes between pH 8.3 and 10.0, which covers it.
7	pH 1.00	The hydrogen ion concentration is 0.050 times two, or 0.100, whose negative logarithm is 1.00.
8	85.0%	The acid totals 0.0100 mol and the back titration uses 0.00150 mol, so 0.00850 mol reacted, meaning 0.00425 mol or 0.425 g of calcium carbonate.
9	pH 12.30	The alkali is in excess by 0.00100 mol in 0.0500 dm^3, giving 0.0200 mol dm^-3 of hydroxide and a pOH of 1.70.
10	It makes no difference; the moles being titrated are unchanged	Adding water only dilutes the solution; the number of moles being titrated is unchanged, so the volume needed stays the same.

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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