JAMB Chemistry 1986

31 reviewed questions with answers and explanations.

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Question 1

The movement of molecules from the surface of a liquid into the gas phase above it is called

  1. Brownian movement
  2. Condensation
  3. Evaporation
  4. Liquefaction
Answer and explanation

C: Evaporation

Evaporation occurs when molecules escape from a liquid surface into the gas phase. Condensation is the reverse change, from gas to liquid.

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Question 3

At the same temperature and pressure, 10 cm³ of hydrogen fluoride reacts with 5 cm³ of dinitrogen difluoride (N₂F₂) to form 10 cm³ of a single gaseous product. Which equation fits these observations?

  1. HF + N₂F₂ → N₂HF₃
  2. 2HF + N₂F₂ → 2NHF₂
  3. 2HF + N₂F₂ → N₂H₂F₄
  4. HF + 2N₂F₂ → N₄HF₄
Answer and explanation

B: 2HF + N₂F₂ → 2NHF₂

At the same temperature and pressure, gas volumes are proportional to mole amounts. The ratio 10:5:10 simplifies to 2:1:2. Equation B has those coefficients and conserves two H atoms, two N atoms and four F atoms on each side.

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Question 4

How many chlorine atoms are present in 5.85 g of NaCl? [Na = 23, Cl = 35.5; Avogadro constant = 6.02 × 10²³ mol⁻¹]

  1. 6.02 × 10²²
  2. 5.85 × 10²³
  3. 6.02 × 10²³
  4. 5.85 × 10²⁴
Answer and explanation

A: 6.02 × 10²²

The molar mass of NaCl is 23 + 35.5 = 58.5 g mol⁻¹. Therefore 5.85 g contains 0.100 mol NaCl. Each formula unit contains one chlorine atom, so the number of chlorine atoms is 0.100 × 6.02 × 10²³ = 6.02 × 10²².

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Question 5

What mass of magnesium reacts completely with 250 cm³ of 0.5 mol dm⁻³ HCl? [Mg = 24]

  1. 0.3 g
  2. 1.5 g
  3. 2.4 g
  4. 3.0 g
Answer and explanation

B: 1.5 g

Mg + 2HCl → MgCl₂ + H₂. The acid amount is 0.250 × 0.5 = 0.125 mol. Half as many moles of magnesium are needed: 0.0625 mol. Its mass is 0.0625 × 24 = 1.5 g.

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Question 8

An element of atomic number 12 commonly forms compounds in which its bonding and valency are

  1. Electrovalent, with valency 1
  2. Electrovalent, with valency 2
  3. Covalent, with valency 2
  4. Covalent, with valency 4
Answer and explanation

B: Electrovalent, with valency 2

Atomic number 12 identifies magnesium. Its electron arrangement is 2,8,2, and it commonly loses two outer electrons to form Mg²⁺. It therefore forms ionic (electrovalent) compounds with valency 2.

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Question 10

Evaporating 50 cm³ of a saturated sugar solution at 40 °C gives 34.2 g of dry sugar. The sugar molar mass is 342.0 g mol⁻¹. What is the saturated concentration at 40 °C?

  1. 10.0 mol dm⁻³
  2. 7.0 mol dm⁻³
  3. 3.5 mol dm⁻³
  4. 2.0 mol dm⁻³
Answer and explanation

D: 2.0 mol dm⁻³

The dissolved sugar amount is 34.2/342.0 = 0.100 mol. The original solution volume is 50/1000 = 0.050 dm³. Its saturated concentration is therefore 0.100/0.050 = 2.0 mol dm⁻³.

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Question 13

Which compound is an acid salt?

  1. NaHSO₄
  2. Na₂SO₄
  3. CH₃CO₂Na
  4. Na₂S
Answer and explanation

A: NaHSO₄

An acid salt retains replaceable hydrogen from partial neutralisation of a polyprotic acid. NaHSO₄ retains one acidic hydrogen from H₂SO₄. The other listed salts do not retain such a hydrogen. Acidity of a salt solution alone is a different concept.

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Question 14

Which aqueous solution has the lowest electrical conductivity?

  1. 2.00 mol dm⁻³ NaOH
  2. 0.01 mol dm⁻³ NaOH
  3. 0.01 mol dm⁻³ hexanoic acid
  4. 0.01 mol dm⁻³ sugar
Answer and explanation

D: 0.01 mol dm⁻³ sugar

Sugar dissolves mainly as neutral molecules and contributes essentially no mobile ions. Sodium hydroxide is a strong electrolyte, while hexanoic acid supplies some ions through partial ionisation. The sugar solution therefore conducts the least electricity.

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Question 16

What charge passes through a solution when a current of 6.5 A flows for 1.0 hour?

  1. 3.90 × 10² C
  2. 5.50 × 10³ C
  3. 6.54 × 10³ C
  4. 2.34 × 10⁴ C
Answer and explanation

D: 2.34 × 10⁴ C

Charge equals current multiplied by time: Q = It. One hour is 3600 seconds, so Q = 6.5 × 3600 = 23400 C = 2.34 × 10⁴ C.

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Question 17

Which equation represents a redox reaction?

  1. AgNO₃ + NaCl → AgCl + NaNO₃
  2. H₂S + Pb(NO₃)₂ → PbS + 2HNO₃
  3. CaCO₃ → CaO + CO₂
  4. Zn + 2HCl → ZnCl₂ + H₂
Answer and explanation

D: Zn + 2HCl → ZnCl₂ + H₂

In Zn + 2HCl → ZnCl₂ + H₂, zinc changes oxidation state from 0 to +2 and hydrogen from +1 to 0. Electrons are transferred, making this a redox reaction. The other reactions do not change the elements’ oxidation states.

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Question 19

Neutralising 20 cm³ of 0.1 mol dm⁻³ aqueous NH₄OH with 20.05 cm³ of 0.1 mol dm⁻³ HCl releases 102 J of heat. What is the molar enthalpy of neutralisation of the base?

  1. −51.0 kJ mol⁻¹
  2. +57.3 kJ mol⁻¹
  3. +57.0 kJ mol⁻¹
  4. +51.0 kJ mol⁻¹
Answer and explanation

A: −51.0 kJ mol⁻¹

The base amount is 0.020 × 0.1 = 0.0020 mol. Since 102 J is released, the reaction enthalpy is negative: ΔH = −102/0.0020 = −51000 J mol⁻¹ = −51.0 kJ mol⁻¹.

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Question 21

Approximately what volume of air contains 10 cm³ of oxygen?

  1. 20 cm³
  2. 25 cm³
  3. 50 cm³
  4. 100 cm³
Answer and explanation

C: 50 cm³

Air is approximately one-fifth oxygen by volume. The air volume is therefore about 10 ÷ 0.20 = 50 cm³. This is an approximate composition calculation.

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Question 22

Which condition gives the reaction Mg + H₂O → MgO + H₂?

  1. Excess magnesium ribbon
  2. Excess cold water
  3. Very hot liquid water
  4. Steam
Answer and explanation

D: Steam

Heated magnesium reacts with steam to produce magnesium oxide and hydrogen: Mg + H₂O(g) → MgO + H₂. Reaction with liquid water instead leads to magnesium hydroxide, so steam matches the stated oxide product.

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Question 23

Which main gaseous products form when steam is passed over red-hot carbon?

  1. Hydrogen, oxygen and carbon dioxide
  2. Hydrogen and carbon dioxide
  3. Hydrogen and carbon monoxide
  4. Hydrogen and carbonic acid
Answer and explanation

C: Hydrogen and carbon monoxide

Hot carbon reduces steam: C + H₂O(g) → CO + H₂. The resulting mixture of carbon monoxide and hydrogen is called water gas.

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Question 27

Why can oxygen-demanding wastes pollute water?

  1. They deplete oxygen needed by aquatic organisms
  2. They increase oxygen needed by aquatic organisms
  3. They increase other gases needed by aquatic organisms
  4. They deplete other gases needed by aquatic organisms
Answer and explanation

A: They deplete oxygen needed by aquatic organisms

Microorganisms use dissolved oxygen while breaking down oxygen-demanding organic waste. If oxygen is consumed faster than it is replaced, less remains for fish and other aquatic organisms.

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Question 28

In which pair can both oxides react further with oxygen to form higher oxides?

  1. NO and H₂O
  2. CO and CO₂
  3. SO₂ and NO
  4. CO₂ and H₂O
Answer and explanation

C: SO₂ and NO

Both gases can be oxidised further: 2SO₂ + O₂ → 2SO₃ and 2NO + O₂ → 2NO₂. Sulfur rises from oxidation state +4 to +6 and nitrogen from +2 to +4. The other pairs contain water or carbon dioxide, which do not undergo this ordinary further oxidation.

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Question 29

Gas X blackens moist lead(II) ethanoate paper, while gas Y bleaches moist litmus paper. Which listed elements are present in X and Y respectively?

  1. H and S; Cl
  2. H and O; Cl
  3. H and S; C and O
  4. H and Cl; S and O
Answer and explanation

A: H and S; Cl

Hydrogen sulfide contains hydrogen and sulfur and forms black lead sulfide with lead(II) ethanoate. Chlorine contains only chlorine and bleaches moist litmus. These observations match the first pair.

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Question 30

Which sulfide is insoluble in dilute hydrochloric acid?

  1. Na₂S
  2. ZnS
  3. CuS
  4. FeS
Answer and explanation

C: CuS

Copper(II) sulfide is extremely insoluble and remains precipitated in dilute non-oxidising acid. Sodium sulfide, zinc sulfide and iron(II) sulfide react with dilute HCl to release hydrogen sulfide. Thus CuS is the acid-insoluble sulfide in the list.

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Question 31

Chlorine water is exposed to sunlight. Which gas is evolved?

  1. HCl
  2. HOCl
  3. O₂
  4. Cl₂O₂
Answer and explanation

C: O₂

Chlorine in water forms hydrochloric acid and hypochlorous acid. Sunlight promotes decomposition of hypochlorous acid, with overall reaction 2HOCl → 2HCl + O₂. The evolved gas is oxygen.

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Question 32

Which metal does not form a stable simple carbonate under ordinary aqueous preparation conditions?

  1. Fe
  2. Al
  3. Zn
  4. Pb
Answer and explanation

B: Al

Aluminium ions strongly hydrolyse carbonate in water, producing aluminium hydroxide and carbon dioxide rather than a stable simple aluminium carbonate. Iron(II), zinc and lead(II) form ordinary solid carbonates.

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Question 34

Which substance is normally used to dry ammonia gas?

  1. Concentrated sulfuric acid
  2. Quicklime
  3. Anhydrous calcium chloride
  4. Magnesium sulfate
Answer and explanation

B: Quicklime

Quicklime, CaO, removes water by forming Ca(OH)₂ without consuming the ammonia. Ammonia reacts with common alternative drying agents such as concentrated sulfuric acid and calcium chloride, so quicklime is used.

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Question 35

What are x, y and z respectively in the balanced equation xCu + yHNO₃ → xCu(NO₃)₂ + 4H₂O + zNO?

  1. 4, 1, 2
  2. 3, 8, 2
  3. 2, 8, 3
  4. 8, 3, 2
Answer and explanation

B: 3, 8, 2

The four water molecules require eight hydrogen atoms, so y = 8. Nitrogen balance then gives 8 = 2x + z. Oxygen balance gives 24 = 6x + 4 + z. Subtracting these equations gives x = 3, then z = 2. The balanced equation is 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 4H₂O + 2NO.

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Question 36

How is iron(III) oxide impurity separated during purification of bauxite?

  1. Fractional crystallisation in acid solution
  2. Treatment with sodium hydroxide followed by filtration
  3. Extraction with concentrated ammonia followed by reprecipitation
  4. Electrolysis of the molten mixture
Answer and explanation

B: Treatment with sodium hydroxide followed by filtration

Hot sodium hydroxide dissolves the aluminium-containing component as aluminate. The iron(III) oxide impurity remains insoluble and can be filtered off. This separates the impurity without dissolving it along with the aluminium.

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Question 39

Which compound gives a lilac flame and a white precipitate with acidified barium chloride solution?

  1. KCl
  2. NaNO₃
  3. K₂SO₄
  4. CaSO₄
Answer and explanation

C: K₂SO₄

Potassium ions give a lilac flame. Sulfate ions form a white barium sulfate precipitate with acidified barium chloride. K₂SO₄ provides both ions, while each other option fails at least one test.

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Question 40

Which extraction method is appropriate for a highly reactive metal that reacts violently with air and dilute acids?

  1. Electrolysis of an aqueous salt solution
  2. Thermal decomposition of its oxide
  3. Displacement from an aqueous solution by an alkali metal
  4. Electrolysis of a molten salt
Answer and explanation

D: Electrolysis of a molten salt

A very reactive metal is obtained by reducing its ions during electrolysis of a molten salt. In an aqueous salt solution, water can be reduced instead, and any highly reactive metal formed would react with the water.

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Question 42

Which compound was historically used as an antiknock additive in petrol for automobile engines?

  1. Tetramethylsilane
  2. Tetraethyllead
  3. Glycerol
  4. n-Heptane
Answer and explanation

B: Tetraethyllead

Tetraethyllead was historically added to petrol to suppress engine knocking by altering combustion chemistry. It is the antiknock additive among these choices.

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Question 43

What reaction produces soap when palm oil is treated with an alkaline potash solution?

  1. Neutralisation
  2. Saponification
  3. Etherification
  4. Salting out
Answer and explanation

B: Saponification

Saponification is the alkaline hydrolysis of fats and oils. Palm oil reacts with the alkaline solution to produce glycerol and salts of fatty acids; the fatty-acid salts are soap.

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Question 47

Which compound is not a direct substitution product when chlorine replaces hydrogen atoms in methane?

  1. CH₃Cl
  2. C₂H₅Cl
  3. CH₂Cl₂
  4. CHCl₃
Answer and explanation

B: C₂H₅Cl

Stepwise replacement of methane hydrogen atoms by chlorine gives CH₃Cl, CH₂Cl₂, CHCl₃ and eventually CCl₄. These direct substitution products retain one carbon atom. C₂H₅Cl has two carbon atoms and is not one of them.

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Question 48

What is the general formula of a saturated acyclic alkyl monohalide, where X is a halogen atom?

  1. CₙH₂ₙ₋₂X
  2. CₙH₂ₙ₊₁X
  3. CₙH₂ₙ₊₂X
  4. CₙH₂ₙX
Answer and explanation

B: CₙH₂ₙ₊₁X

An acyclic alkane has formula CₙH₂ₙ₊₂. Replacing one hydrogen atom with a halogen atom X gives CₙH₂ₙ₊₁X, the formula for a saturated acyclic monohaloalkane.

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Question 49

Which pair consists of materials made by polymerisation?

  1. Nylon and soap
  2. Nylon and rubber
  3. Soap and butane
  4. Margarine and nylon
Answer and explanation

B: Nylon and rubber

Nylon and rubber are polymers made by linking many small molecular units. Nylon can form by condensation polymerisation, while common synthetic rubbers form from unsaturated monomers. Soap and margarine are produced by different reactions.

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Question 50

After starch has been hydrolysed to sugars, which process converts those sugars into ethyl alcohol?

  1. Distillation
  2. Fermentation
  3. Isomerisation
  4. Cracking
Answer and explanation

B: Fermentation

Starch is first hydrolysed to fermentable sugars. Yeast then converts glucose into ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Distillation can subsequently separate the ethanol but does not create it.

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