JAMB Chemistry 2002

33 reviewed questions with answers and explanations.

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Question 1

The formula CH₂O for ethanoic acid is its

  1. Empirical formula
  2. Molecular formula
  3. Structural formula
  4. General formula
Answer and explanation

A: Empirical formula

Ethanoic acid has molecular formula C₂H₄O₂. Dividing all subscripts by two gives the simplest whole-number ratio CH₂O, its empirical formula.

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Question 2

Which gas sample contains the fewest atoms at standard temperature and pressure?

  1. 7 mol argon
  2. 4 mol chlorine
  3. 3 mol ozone
  4. 1 mol butane
Answer and explanation

A: 7 mol argon

Count moles of atoms: argon gives 7 × 1 = 7; chlorine gives 4 × 2 = 8; ozone gives 3 × 3 = 9; butane gives 1 × (4 + 10) = 14. The argon sample therefore contains the fewest atoms.

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Question 3

Chromatographic separation of ink is based on the ability of its components to

  1. Dissolve in each other in the column
  2. Move at different speeds in the column
  3. React with the solvent
  4. React with each other
Answer and explanation

B: Move at different speeds in the column

Ink components have different affinities for the stationary phase and the moving solvent. They therefore travel at different rates and separate into bands; chemical reaction between the components is not required.

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Question 6

Which gas forms the complex responsible for the brown colour in the brown-ring test?

  1. CO
  2. NO
  3. CO₂
  4. NO₂
Answer and explanation

B: NO

Nitric oxide, NO, binds to iron in the nitrosyl complex responsible for the brown ring. The colour belongs to the complex; free NO gas is colourless.

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Question 7

Which aqueous solution forms a precipitate when a small amount of NaOH solution is added?

  1. NH₄Cl
  2. Na₂CO₃
  3. AlCl₃
  4. CH₃COONa
Answer and explanation

C: AlCl₃

Adding a limited amount of hydroxide to aluminium chloride forms white Al(OH)₃: Al³⁺ + 3OH⁻ → Al(OH)₃(s). The precipitate dissolves in excess NaOH because aluminium hydroxide is amphoteric.

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Question 8

The reaction of an alkene with hydrogen in the presence of a catalyst is

  1. A nucleophilic reaction
  2. An addition reaction
  3. A substitution reaction
  4. An oxidative reaction
Answer and explanation

B: An addition reaction

Hydrogen adds across the alkene C=C bond in the presence of a suitable catalyst. The carbon skeleton remains intact and a saturated alkane forms, so this is an addition reaction.

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Question 10

The intermediate formed when ethanol is progressively oxidised to ethanoic acid using acidified potassium dichromate is

  1. Methanal
  2. Propanal
  3. Ethanal
  4. Butanal
Answer and explanation

C: Ethanal

Ethanol first oxidises to ethanal, CH₃CHO. Further oxidation converts the aldehyde group into the carboxylic acid group of ethanoic acid.

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Question 11

CH₃CH₂CH(OH)CH₃ is a

  1. Primary alkanol
  2. Secondary alkanol
  3. Tertiary alkanol
  4. Glycol
Answer and explanation

B: Secondary alkanol

In CH₃CH₂CH(OH)CH₃, the carbon bearing OH is bonded to two other carbon atoms and one hydrogen. It is therefore a secondary alcohol, butan-2-ol.

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Question 15

What is the specific name for boiling fat with aqueous caustic soda to produce soap?

  1. Acidification
  2. Hydrolysis
  3. Saponification
  4. Esterification
Answer and explanation

C: Saponification

Boiling a fat with aqueous sodium hydroxide breaks its ester bonds to form glycerol and sodium salts of fatty acids, which are soaps. This specific alkaline hydrolysis is called saponification.

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Question 16

Ordinary soda-lime glass is manufactured from silica, CaCO₃ and

  1. NaHCO₃
  2. K₂SO₄
  3. K₂CO₃
  4. Na₂CO₃
Answer and explanation

D: Na₂CO₃

Soda-lime glass is made using silica, limestone and sodium carbonate. Sodium carbonate supplies sodium oxide in the melt and acts as a flux, lowering the temperature needed to process silica.

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Question 20

Which listed element is especially noted for extensive catenation into long chains and rings?

  1. Nitrogen
  2. Chlorine
  3. Carbon
  4. Bromine
Answer and explanation

C: Carbon

Carbon forms strong covalent bonds with other carbon atoms, supporting extensive chains, branched structures and rings. This extensive catenation is central to organic chemistry.

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Question 21

Ethanol can be produced from starch by

  1. Distilling a starch solution
  2. Catalytic oxidation of methane
  3. Destructive distillation of wood
  4. Fermentation after starch hydrolysis
Answer and explanation

D: Fermentation after starch hydrolysis

Starch is first hydrolysed into fermentable sugars. Yeast can then convert those sugars into ethanol and carbon dioxide. Distillation separates ethanol after production but does not convert starch into ethanol.

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Question 22

Hydrogen is readily released when dilute hydrochloric acid reacts with

  1. Ag
  2. Au
  3. Cu
  4. Na
Answer and explanation

D: Na

Sodium is sufficiently reactive to release hydrogen from aqueous hydrochloric acid, producing sodium chloride. Silver, gold and copper do not displace hydrogen from dilute non-oxidising hydrochloric acid under ordinary conditions.

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Question 23

Which statement about a proton is correct?

  1. Its mass is 1.0008 g
  2. Its mass is one-twelfth the molar mass of carbon
  3. Its mass is approximately 1840 times the mass of an electron
  4. The total mass of the protons in any nucleus is always half its nuclear mass
Answer and explanation

C: Its mass is approximately 1840 times the mass of an electron

A proton is about 1836 times as massive as an electron, often rounded to 1840 in school calculations. Its mass is about 1.67 × 10⁻²⁴ g. A molar mass describes one mole, not one particle, and nuclei do not all have equal numbers of protons and neutrons.

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Question 24

In the beta-minus decay ¹⁴₆C → X + β⁻, which nuclide is X?

  1. ¹⁴₇N
  2. ¹³₆C
  3. ¹²₆C
  4. ¹²₅B
Answer and explanation

A: ¹⁴₇N

In beta-minus decay a neutron changes into a proton, so atomic number increases by one while mass number stays the same. Carbon-14, with atomic number 6, becomes nitrogen-14 with atomic number 7.

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Question 26

Which chloride has the least ionic character?

  1. LiCl
  2. MgCl₂
  3. CaCl₂
  4. AlCl₃
Answer and explanation

D: AlCl₃

Al³⁺ has high charge and small size, giving strong polarising power. It distorts the chloride electron cloud, increasing covalent character; AlCl₃ is therefore the least ionic of these choices.

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Question 28

Processes that return carbon dioxide to the atmosphere include

  1. Photosynthesis, respiration and transpiration
  2. Respiration, decay and combustion
  3. Photosynthesis, decay and respiration
  4. Ozone depletion, combustion and decay
Answer and explanation

B: Respiration, decay and combustion

Respiration, aerobic decomposition and combustion release carbon dioxide. Photosynthesis instead takes up CO₂, and transpiration chiefly releases water vapour.

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Question 29

Which Dalton postulate is retained in the modern description of stoichiometric compounds?

  1. All elements consist of indivisible particles
  2. Atoms of different elements combine in simple whole-number ratios
  3. Atoms can never be created or destroyed in any process
  4. All atoms of the same element are exactly alike
Answer and explanation

B: Atoms of different elements combine in simple whole-number ratios

Atoms combine in whole-number ratios in the formulas of stoichiometric compounds. Atoms have subatomic structure, isotopes of an element need not be identical in mass, and nuclear processes can transform atoms, so those absolute older claims require revision.

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Question 30

A non-reacting ideal-gas mixture contains 0.75 mol cyclopropane and 0.66 mol oxygen at total pressure 0.7 atm. What is the partial pressure of oxygen?

  1. 0.22 atm
  2. 0.33 atm
  3. 0.44 atm
  4. 0.55 atm
Answer and explanation

B: 0.33 atm

The oxygen mole fraction is 0.66/(0.75 + 0.66) = 0.4681. Dalton’s law gives p(O₂) = 0.4681 × 0.7 = 0.3277 atm, or 0.33 atm.

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Question 31

When H₂S reduces aqueous iron(III) chloride, the resulting iron-ion solution is

  1. Brown
  2. Pale green
  3. Colourless
  4. Pale red
Answer and explanation

B: Pale green

Hydrogen sulfide reduces Fe³⁺ to Fe²⁺, whose aqueous solution is pale green: 2Fe³⁺ + H₂S → 2Fe²⁺ + S + 2H⁺. Sulfur also separates as a solid.

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Question 32

Which condition specifically indicates chemical equilibrium at fixed temperature and pressure? Here ΔᵣG is the reaction Gibbs energy at the current composition.

  1. ΔᵣG = ΔᵣH − TΔᵣS
  2. ΔᵣG < 0
  3. ΔᵣG = 0
  4. ΔᵣG > 0
Answer and explanation

C: ΔᵣG = 0

At equilibrium, the reaction Gibbs energy ΔᵣG is zero, so neither direction has a thermodynamic driving force. This does not require the standard reaction Gibbs energy ΔᵣG° to be zero.

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Question 33

For Cu₂S(s) + O₂(g) → 2Cu(s) + SO₂(g), how does copper’s oxidation number change?

  1. 0 to +2
  2. 0 to +1
  3. +1 to 0
  4. +2 to +1
Answer and explanation

C: +1 to 0

Sulfide has oxidation number −2 in Cu₂S, so each copper is +1. Elemental copper has oxidation number 0, giving the change +1 → 0.

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Question 36

Electrolysis of dilute aqueous sodium hydroxide using platinum electrodes produces

  1. Sodium metal and oxygen gas
  2. Hydrogen and oxygen gases
  3. Water and hydrogen gas
  4. Water and sodium metal
Answer and explanation

B: Hydrogen and oxygen gases

With inert platinum electrodes, water is reduced to hydrogen at the cathode and hydroxide is oxidised to oxygen at the anode. The net change is 2H₂O → 2H₂ + O₂.

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Question 37

For PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), which statement describes the equilibrium shift when pressure is decreased by expansion at fixed temperature?

  1. The yield of PCl₃ increases
  2. The yield of PCl₅ increases
  3. The reaction is accelerated
  4. The reaction is decelerated
Answer and explanation

A: The yield of PCl₃ increases

The product side has two moles of gas for each mole on the reactant side. Expansion at fixed temperature lowers pressure and favours the products, increasing the equilibrium amount of PCl₃.

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Question 38

The Arrhenius equation relates a reaction’s rate constant to temperature and its

  1. Catalyst
  2. Activation energy
  3. Molecular collisions
  4. Heat of reaction
Answer and explanation

B: Activation energy

The Arrhenius equation is k = A exp(−Eₐ/RT). It relates the rate constant to activation energy and temperature, with A the pre-exponential factor. At fixed A and T, a lower activation energy gives a larger rate constant.

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Question 40

Which description gives the thermal effect when NaOH flakes dissolve in water?

  1. A rapid reaction
  2. A slow reaction
  3. An exothermic change
  4. An endothermic change
Answer and explanation

C: An exothermic change

Dissolving sodium hydroxide releases heat overall because hydration of its ions releases more energy than is needed to separate the solid lattice. The solution warms, so the change is exothermic.

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Question 41

Water vapour changes anhydrous cobalt(II) chloride from

  1. Blue to white
  2. White to green
  3. Blue to pink
  4. White to red
Answer and explanation

C: Blue to pink

Anhydrous cobalt(II) chloride is blue. On taking up water it forms hydrated species that appear pink, producing the blue-to-pink change used in a water test.

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Question 43

At 20 °C, a salt of molar mass 101 g mol⁻¹ has solubility 0.34 mol dm⁻³. If 3.40 g dissolves in 250 cm³ of water with negligible volume change, the solution is

  1. Saturated
  2. Unsaturated
  3. Supersaturated
  4. A suspension
Answer and explanation

B: Unsaturated

The amount is 3.40/101 = 0.0337 mol. With negligible volume change, concentration is 0.0337/0.250 = 0.135 mol dm⁻³, below the solubility of 0.34 mol dm⁻³. More salt can dissolve, so the solution is unsaturated.

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Question 44

25 cm³ of 0.2 mol dm⁻³ Na₂CO₃ requires 20 cm³ of HCl for complete neutralisation. What is the HCl concentration?

  1. 0.2 mol dm⁻³
  2. 0.4 mol dm⁻³
  3. 0.5 mol dm⁻³
  4. 0.6 mol dm⁻³
Answer and explanation

C: 0.5 mol dm⁻³

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. The carbonate amount is 0.025 × 0.2 = 0.005 mol, requiring 0.010 mol HCl. The acid concentration is 0.010/0.020 = 0.50 mol dm⁻³.

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Question 45

When a hydrated salt loses water of crystallisation on exposure to air, the process is called

  1. Effervescence
  2. Efflorescence
  3. Fluorescence
  4. Deliquescence
Answer and explanation

B: Efflorescence

Efflorescence is the loss of water of crystallisation from a hydrated salt on exposure to air. Deliquescence instead involves absorbing moisture until the solid dissolves.

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Question 46

Three drops of 1.0 mol dm⁻³ NaOH are added to 20 cm³ of an aqueous solution at pH 8.4. The resulting pH will be

  1. Less than 8.4
  2. Greater than 8.4
  3. Unaltered
  4. Close to that of pure water
Answer and explanation

B: Greater than 8.4

Adding concentrated sodium hydroxide increases the hydroxide concentration and decreases the hydrogen-ion concentration. The resulting pH is greater than 8.4.

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Question 49

Which interaction chiefly explains why a small alcohol such as ethanol mixes readily with water?

  1. Ionic character
  2. Boiling point
  3. Covalent nature
  4. Hydrogen bonding
Answer and explanation

D: Hydrogen bonding

The OH group in a small alcohol such as ethanol can form hydrogen bonds with water. This favours mixing; solubility decreases as the non-polar carbon chain becomes longer.

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Question 50

Calcium carbonate scale formed when temporary hard water is boiled comes from dissolved

  1. Calcium hydrogencarbonate
  2. Calcium carbonate
  3. Calcium sulfate
  4. Calcium hydroxide
Answer and explanation

A: Calcium hydrogencarbonate

On heating, dissolved calcium hydrogencarbonate decomposes: Ca(HCO₃)₂ → CaCO₃(s) + CO₂ + H₂O. Insoluble calcium carbonate coats the kettle as scale.

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