5 high-yield, syllabus-aligned questions on Acids, bases & salts, covering 15 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
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Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
State4 marks
State the Brønsted–Lowry definitions of an acid and a base, and give one example of each.
TopMarks model answer
A Brønsted–Lowry acid is a species that donates a proton, that is a hydrogen ion, to another species; hydrogen chloride is an example. A Brønsted–Lowry base is a species that accepts a proton from another species; ammonia is an example.
Suggested marking guide
1Acid donates a proton
2A valid acid example
3Base accepts a proton
4A valid base example
Why it scores
The words DONATES and ACCEPTS are the mark scheme. “Gives out hydrogen ions” describes the classical Arrhenius definition and is not credited under a Brønsted–Lowry question. And each example is a mark of its own, so an answer with two perfect definitions and no examples scores half.
A common incomplete answer
“An acid donates a proton and a base accepts a proton.”
What it costs: Two marks of four. Both definitions are exactly right, but the question asked for one example of each and neither was supplied.
Explain what is meant by a normal salt, an acid salt and a basic salt, giving one example of each.
TopMarks model answer
A normal salt is formed when all the ionisable hydrogen of an acid has been completely replaced by a metal or by the ammonium ion; sodium chloride is an example. An acid salt is formed when only part of the ionisable hydrogen of a polybasic acid has been replaced; sodium hydrogen trioxocarbonate(IV) is an example. A basic salt contains hydroxide or oxide ions in addition to the normal anion of the salt; basic lead trioxonitrate(V) is an example.
Suggested marking guide
1Normal salt: all ionisable hydrogen replaced, with an example
2Acid salt: only part replaced, with an example
3Basic salt: contains hydroxide or oxide as well, with an example
Why it scores
Only a POLYBASIC acid can give an acid salt, because there must be more than one replaceable hydrogen for part of it to remain. Saying an acid salt “still has hydrogen in it” is not enough — the hydrogen must be IONISABLE hydrogen from the acid, which is why ethanoic acid, CH₃COOH, forms no acid salt despite having four hydrogen atoms.
A common incomplete answer
“A normal salt has all its ionisable hydrogen replaced by a metal, for example sodium chloride.”
What it costs: Two marks of three. The normal salt is defined and exemplified correctly, but acid salts and basic salts are not attempted.
Key ideas to include: ionisable hydrogen, polybasic acid, normal salt, acid salt, basic salt.
Describe4 marks
Describe how you would prepare crystals of sodium chloride in the laboratory, starting from sodium hydroxide solution.
TopMarks model answer
Measure a known volume of the sodium hydroxide solution into a conical flask and add two drops of methyl orange. Run dilute hydrochloric acid from a burette until the indicator just changes colour, and note the volume used. Repeat the titration using the same volumes but without adding the indicator, so that the salt is not contaminated by it. Evaporate the resulting solution over a water bath until it is saturated, allow it to cool slowly so that crystals form, then filter and dry the crystals between filter papers.
Suggested marking guide
1Titrate with an indicator to find the exact volume
2Repeat without the indicator to avoid contamination
3Evaporate to the point of saturation, not to dryness
4Cool to crystallise, then dry the crystals
Why it scores
Two of these four marks are for details that look like fussiness and are not. Repeating WITHOUT the indicator matters because the indicator is a dye that would colour the crystals. Evaporating to SATURATION rather than to dryness matters because boiling dry gives a powder, not crystals. Examiners award marks for exactly these two points because they show the candidate has done the experiment rather than read about it.
A common incomplete answer
“Titrate the sodium hydroxide with hydrochloric acid using methyl orange, then evaporate the solution to crystallisation and allow it to cool.”
What it costs: Two marks of four. The titration and the evaporation are credited. Repeating without the indicator is missing, so the crystals would be contaminated, and the crystals are never filtered and dried.
Key ideas to include: titration, burette, indicator, evaporate to saturation, crystallise, water bath.
Explain2 marks
Explain why an aqueous solution of sodium trioxocarbonate(IV) is alkaline.
TopMarks model answer
Sodium trioxocarbonate(IV) is the salt of a strong base, sodium hydroxide, and a weak acid, trioxocarbonic(IV) acid. In water the trioxocarbonate ion is hydrolysed, accepting protons from water molecules and releasing hydroxide ions, so the concentration of hydroxide ions exceeds that of hydrogen ions and the solution is alkaline.
Suggested marking guide
1Salt of a strong base and a weak acid
2Hydrolysis releases hydroxide ions
Why it scores
The rule is worth memorising because it answers a whole family of questions: strong base plus weak acid gives an alkaline salt solution; strong acid plus weak base gives an acidic one; strong plus strong gives a neutral one. Both marks here are for showing that you applied the rule, not merely that you know the outcome.
A common incomplete answer
“Because sodium trioxocarbonate(IV) is a base.”
What it costs: Both marks. It is a SALT, not a base. The alkalinity comes from hydrolysis of its anion, and neither the parent acid and base nor the hydrolysis is mentioned.
Distinguish between a strong acid and a concentrated acid.
TopMarks model answer
A strong acid is one that ionises completely in aqueous solution to give a high concentration of hydrogen ions; strength refers to the degree of ionisation. A concentrated acid is one that contains a large amount of the acid in a small volume of solution; concentration refers to the amount of solute per unit volume.
Suggested marking guide
1Strength is the degree of ionisation
2Concentration is the amount of solute per unit volume
Why it scores
These two words are not synonyms and the examiner is testing exactly that. Ethanoic acid can be concentrated but is always weak; hydrochloric acid can be dilute but is always strong. The distinction is ionisation versus amount, and each half is worth a mark.
A common incomplete answer
“A strong acid ionises completely in water while a concentrated acid has a lot of acid in it.”
What it costs: One mark of two. The strength half is exactly right. “A lot of acid in it” does not say per unit volume, so it does not define concentration.
Key ideas to include: degree of ionisation, completely ionised, amount of solute, per unit volume.
Practise Acids, bases & salts on real questions
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