Laboratory preparation of gases — questions and answers
4 high-yield, syllabus-aligned questions on Laboratory preparation of gases, 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 four precautions that must be observed during the laboratory preparation of a gas.
TopMarks model answer
All the apparatus must be dry and the joints airtight, so that the gas is neither contaminated nor lost. The first portions of gas collected must be discarded, because they are mixed with the air already in the apparatus. Toxic gases such as chlorine and sulphur(IV) oxide must be prepared in a fume cupboard. When a solid is being heated in a test tube, the tube must be inclined with its mouth downwards, and the delivery tube must be removed from the water before the flame is withdrawn, to prevent suck-back.
Suggested marking guide
1Apparatus dry and airtight
2Discard the first portions of gas
3Toxic gases prepared in a fume cupboard
4Prevent suck-back
Why it scores
These are marks for laboratory judgement rather than chemical knowledge, which makes them the most reliably earnable marks in any practical question — and the most consistently ignored. Learn all four; they apply to almost every gas on the syllabus.
A common incomplete answer
“The apparatus must be airtight and the experiment must be done carefully.”
What it costs: Three marks of four. The airtight apparatus is credited. “Carefully” is not a precaution; the examiner wants specific, named actions.
Key ideas to include: airtight, first portions, fume cupboard, suck-back, inclined tube.
Describe4 marks
Describe the laboratory preparation of oxygen from hydrogen peroxide and state how the gas is tested.
TopMarks model answer
Place aqueous hydrogen peroxide in a flat-bottomed flask and add a little manganese(IV) oxide. The manganese(IV) oxide acts as a catalyst, and oxygen is evolved without heating: 2H₂O₂ → 2H₂O + O₂. The gas is collected over water, because it is only slightly soluble in water. To test it, introduce a glowing splint into a jar of the gas: the splint rekindles.
Suggested marking guide
1Hydrogen peroxide with manganese(IV) oxide
2Manganese(IV) oxide identified as a catalyst
3Collected over water
4Glowing splint rekindles
Why it scores
Two details carry marks that look like details and are not. Manganese(IV) oxide must be named as a CATALYST — it is not consumed, and saying it “reacts with” the peroxide is wrong. And the splint must be GLOWING, not lighted: a lighted splint burns brighter in oxygen, but the standard test the mark scheme wants is the glowing splint rekindling.
A common incomplete answer
“Add manganese(IV) oxide to hydrogen peroxide and collect the oxygen given off.”
What it costs: Three marks of four. The reagents are credited, but the catalyst is not named as such, the method of collection is missing, and no test is described.
Key ideas to include: hydrogen peroxide, manganese(IV) oxide, catalyst, over water, glowing splint.
Name4 marks
Name the drying agent used for each of the following gases and, in each case, name one common drying agent that must be avoided and say why: (i) hydrogen chloride (ii) ammonia.
TopMarks model answer
Hydrogen chloride is dried with concentrated tetraoxosulphate(VI) acid, which does not react with it. It must not be dried with quicklime, because quicklime is basic and would react with the acidic gas. Ammonia is dried with quicklime, calcium oxide. It must not be dried with concentrated tetraoxosulphate(VI) acid, because ammonia is basic and reacts with the acid, nor with anhydrous calcium chloride, with which it forms a complex.
Suggested marking guide
1Hydrogen chloride dried with concentrated tetraoxosulphate(VI) acid
2Quicklime avoided because it is basic
3Ammonia dried with quicklime
4Calcium chloride avoided because a complex forms
Why it scores
The whole topic reduces to one principle: a drying agent must not REACT with the gas it dries. Acidic gases take acidic drying agents, basic gases take basic ones. Once you hold that rule, you can answer this question for any gas on the syllabus without memorising a table.
A common incomplete answer
“Hydrogen chloride is dried with concentrated tetraoxosulphate(VI) acid and ammonia with quicklime.”
What it costs: Two marks of four. Both drying agents are correct, but the question also asked which agent must be AVOIDED in each case and why, and neither reason is given.
State three methods of collecting gases in the laboratory, giving one gas collected by each method and the reason for the choice.
TopMarks model answer
Collection over water is used for gases that are insoluble or only slightly soluble in water, such as hydrogen and oxygen. Collection by upward delivery, with the mouth of the jar downwards, is used for gases that are lighter than air and soluble in water, such as ammonia. Collection by downward delivery, with the mouth of the jar upwards, is used for gases denser than air and soluble in water, such as hydrogen chloride and chlorine.
Suggested marking guide
1Over water for insoluble gases
2Upward delivery for gases lighter than air
3Downward delivery for gases denser than air
Why it scores
Two properties decide the method every time: solubility in water and density relative to air. Give both in each line and the reason mark comes automatically. Naming the three methods without their reasons scores nothing, because the reason is the whole content of the mark.
A common incomplete answer
“Over water, by upward delivery, and by downward delivery.”
What it costs: All three marks. The methods are correctly named, but the question asked for a gas and a reason in each case, and the mark points require them.
Key ideas to include: over water, upward delivery, downward delivery, solubility, density relative to air.
Practise Laboratory preparation of gases on real questions
Reading a full-mark answer is the first half. Writing one under time is the other. A free account opens exam-standard practice in Chemistry with the full solution on every question.