What is the commonest pH mistake?

Treating pH as a concentration: one pH unit means a tenfold change in hydrogen ion concentration.

Is there a fixed routine for titration sums?

Yes: concentration times volume gives moles, scale by the ratio, then divide by the new volume.

S4 Chemistry: Acids, Bases and Salts (Paper 4)

2026-11-01

Tutor Prep Guide · Secondary Chemistry

This paper mixes pH calculations, titration, dilution and salt preparation, and repairs the common logarithmic error.

Direct answer　Write the hydrogen ion concentration before taking the negative log, and always convert to moles before using the equation ratio.

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: Acids, Bases and Salts
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationFind the pH of 0.010 M nitric acid, a strong acid that ionises completely.
FoundationFind the pH of 0.0050 M sodium hydroxide to one decimal place.
FoundationWhat is the hydrogen ion concentration of a solution of pH 3?
FoundationWhat volume of 0.050 M sodium hydroxide neutralises 25.0 cm3 of 0.10 M hydrochloric acid?
Intermediate20.0 cm3 of 0.15 M sodium hydroxide just neutralises 25.0 cm3 of hydrochloric acid. Find the acid concentration.
Intermediate0.010 M hydrochloric acid is diluted one hundred times. What is the new pH?
IntermediateEqual volumes of hydrochloric acid and sodium hydroxide of the same concentration are mixed. What is the pH?
ChallengeWhat volume of 0.20 M sodium hydroxide neutralises 25.0 cm3 of 0.10 M sulphuric acid, a diprotic acid?
Challenge0.050 mol of barium chloride reacts with excess sodium sulphate. Find the mass of barium sulphate with BaSO4 = 233.
Error redoError repair: a student writes that going from pH 2 to pH 4 means dividing the concentration by 4. Correct the statement.

03　Answers and one-line notes

#	Answer	One-line note
1	pH 2.0	The acid ionises completely so the hydrogen ion concentration is 0.010 mol dm-3 and minus log gives 2.0.
2	pH 11.7	Work out pOH first, minus log of 0.0050 is 2.3, and 14 minus 2.3 gives 11.7.
3	1.0 x 10-3 mol dm-3	pH is the negative log of the hydrogen ion concentration, so pH 3 means 1.0 times ten to the minus three.
4	50.0 cm3	The acid holds 0.0025 mol, the ratio is one to one, and dividing by 0.050 gives 0.050 dm3 or 50.0 cm3.
5	0.120 M	The alkali gives 0.0030 mol, and dividing by the acid volume of 0.0250 dm3 gives 0.120 M.
6	pH 4.0	The concentration falls to 1.0 times ten to the minus four, so the pH rises by two units to 4.0.
7	pH 7, neutral	Equal moles of acid and alkali cancel out completely, leaving water and salt, so the pH is 7.
8	25.0 cm3	The acid gives 0.0050 mol of hydrogen ions because it is diprotic, and dividing by 0.20 gives 0.025 dm3.
9	11.65 g	The ratio is one to one, so 0.050 mol of precipitate forms and 0.050 times 233 gives 11.65 g, which is how an insoluble salt is prepared.
10	each pH unit is a tenfold change, so pH 2 to 4 is a hundredfold dilution	pH is a logarithmic scale, so a gap of two units means a hundredfold change in concentration, not fourfold.

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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