JAMB Chemistry 2003

28 reviewed questions with answers and explanations.

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Question 3

What is the percentage by mass of oxygen in Al₂(SO₄)₃·2H₂O? [Al = 27, S = 32, H = 1, O = 16]

  1. 14.29%
  2. 25.39%
  3. 50.79%
  4. 59.26%
Answer and explanation

D: 59.26%

The formula contains 2 Al, 3 S, 14 O and 4 H atoms. Its molar mass is 54 + 96 + 224 + 4 = 378 g mol⁻¹. Oxygen contributes 224 g, so its percentage is 224/378 × 100 = 59.26% to two decimal places.

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Question 5

In 3Cu + pHNO₃ → 3Cu(NO₃)₂ + 4H₂O + xNO, what are p and x respectively?

  1. 1 and 3
  2. 2 and 3
  3. 6 and 2
  4. 8 and 2
Answer and explanation

D: 8 and 2

Three copper atoms lose six electrons in total. Two nitrate nitrogen atoms each gain three electrons to form NO; six further nitrate groups remain with copper. Balancing H and O gives 3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 4H₂O + 2NO.

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Question 7

Neon and the heavier noble gases owe their low reactivity largely to their

  1. Octet configuration
  2. Cyclic shape
  3. Hexagonal shape
  4. Obtuse configuration
Answer and explanation

A: Octet configuration

Neon and the heavier noble gases have eight outer-shell electrons, a stable octet. Helium is a separate case with its first shell filled by two electrons.

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Question 8

According to kinetic theory, increasing absolute temperature causes average kinetic energy of gas particles to

  1. Decrease
  2. Increase
  3. Remain constant
  4. Be zero
Answer and explanation

B: Increase

Mean translational kinetic energy of gas particles is proportional to absolute temperature: average Eₖ = 3kT/2. Increasing temperature therefore increases their average kinetic energy.

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Question 11

When cathode rays are deflected onto the electrode of an electrometer, the instrument becomes

  1. negatively charged
  2. positively charged
  3. neutral
  4. bipolar
Answer and explanation

A: negatively charged

Cathode rays are streams of electrons. Collection of these electrons by the electrometer electrode adds negative charge.

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Question 12

Which listed interaction is ordinarily the weakest individual attraction?

  1. Ionic bonding
  2. Covalent bonding
  3. Coordinate covalent bonding
  4. Van der Waals attraction
Answer and explanation

D: Van der Waals attraction

Van der Waals interactions are generally much weaker individually than ordinary ionic or covalent bonds. Coordinate covalent bonds are also covalent bonds.

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Question 14

Which ion acts as an acid in water?

  1. K⁺
  2. NO₃⁻
  3. S²⁻
  4. H₃O⁺
Answer and explanation

D: H₃O⁺

Hydronium, H₃O⁺, donates a proton in aqueous acid-base reactions. K⁺ and NO₃⁻ do not appreciably acidify water, while S²⁻ acts as a base.

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Question 18

Which method can biologically help decontaminate farmland affected by crude-oil spillage?

  1. Adding acidic solution
  2. Using suitable aerobic bacteria
  3. Pouring water over the area
  4. Burning the oil off
Answer and explanation

B: Using suitable aerobic bacteria

Suitable aerobic bacteria break down some petroleum hydrocarbons in the presence of oxygen. This bioremediation requires appropriate conditions and does not instantly remove every oil component.

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Question 21

For an equilibrium whose forward reaction has ΔH > 0, which temperature change favours the forward equilibrium direction?

  1. Increasing temperature
  2. Any temperature change
  3. Decreasing temperature
  4. Cooling to a minimum temperature
Answer and explanation

A: Increasing temperature

The forward reaction is endothermic when ΔH is positive. Raising temperature favours the heat-absorbing direction and shifts equilibrium towards products.

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Question 23

The defining feature of an anode reaction is that

  1. Electrons are consumed
  2. Oxidation occurs
  3. Ions are reduced
  4. The electrode dissolves
Answer and explanation

B: Oxidation occurs

Oxidation occurs at the anode: a species loses electrons. An anode may dissolve if it is an active metal, but dissolution is not required for every anode reaction.

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Question 24

Which quantity changes when a catalyst provides an alternative pathway?

  1. Activation energy
  2. Potential energy of reactants
  3. Heat of reaction
  4. Potential energy of products
Answer and explanation

A: Activation energy

A catalyst provides a pathway with lower activation energy. It does not alter the energies of starting reactants and final products, so reaction enthalpy is unchanged.

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Question 25

If Y is an oxidizing agent that reacts with a reducing agent, Z, which of the following is correct?

  1. Y increases in oxidation number
  2. Y becomes reduced
  3. Z loses protons
  4. Z gains protons.
Answer and explanation

B: Y becomes reduced

An oxidising agent accepts electrons and is reduced. Thus Y is reduced while reducing agent Z loses electrons and is oxidised.

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Question 27

In electrolysis of concentrated aqueous NaCl with inert electrodes, which pair represents cathode and anode discharge respectively in introductory ion-discharge notation?

  1. Na⁺ and Cl⁻
  2. Na⁺ and OH⁻
  3. H⁺ and OH⁻
  4. H⁺ and Cl⁻
Answer and explanation

D: H⁺ and Cl⁻

Hydrogen forms at the cathode and chlorine at the anode. Introductory ion-discharge notation describes these as H⁺ and Cl⁻ discharge. The cathode process is more directly written 2H₂O + 2e⁻ → H₂ + 2OH⁻.

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Question 28

For CO(g) + H₂O(g) → CO₂(g) + H₂(g), calculate ΔH° using formation enthalpies CO₂(g) = −394, H₂O(g) = −242 and CO(g) = −110 kJ mol⁻¹.

  1. −262 kJ mol⁻¹
  2. −42 kJ mol⁻¹
  3. +42 kJ mol⁻¹
  4. +262 kJ mol⁻¹
Answer and explanation

B: −42 kJ mol⁻¹

Use products minus reactants: ΔH° = [−394 + 0] − [−110 + (−242)] = −42 kJ mol⁻¹. Hydrogen in its standard elemental state has zero standard enthalpy of formation.

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Question 30

Which listed sample conducts electricity through mobile ions under the stated conditions?

  1. Pure alcohol
  2. Sodium acetate solution
  3. Solid potassium hydroxide
  4. Mercury
Answer and explanation

B: Sodium acetate solution

Sodium acetate solution contains mobile Na⁺ and CH₃COO⁻ ions. Solid KOH has ions fixed in a lattice; mercury conducts through electrons rather than electrolytic ion movement.

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Question 32

Many characteristic transition-metal properties arise from partly filled subshells in their atoms or ions of type

  1. s
  2. p
  3. d
  4. f
Answer and explanation

C: d

Many transition-metal properties arise from incompletely filled d subshells in their atoms or common ions. These states help explain variable oxidation states and many coloured complexes.

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Question 34

Which statement is true of sulfur(IV) oxide, SO₂?

  1. It forms sulfuric acid directly with water
  2. It is an odourless gas
  3. It is an acid anhydride
  4. It gives a white precipitate with acidified barium chloride
Answer and explanation

C: It is an acid anhydride

SO₂ is an acidic oxide, conventionally described as the anhydride of sulfurous acid. It acidifies water and is pungent rather than odourless. Dissolving it in water does not directly produce sulfuric acid.

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Question 35

Which salt solution forms a precipitate with aqueous ammonia that dissolves in excess ammonia?

  1. Ca(NO₃)₂
  2. Cu(NO₃)₂
  3. Mg(NO₃)₂
  4. Al(NO₃)₃
Answer and explanation

B: Cu(NO₃)₂

A little aqueous ammonia produces blue Cu(OH)₂ from Cu²⁺. Excess ammonia dissolves it by forming a deep-blue soluble copper-ammonia complex, commonly written [Cu(NH₃)₄(H₂O)₂]²⁺.

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Question 37

What initial colour does chlorine produce on damp starch-iodide paper?

  1. Pink
  2. Colourless
  3. Red
  4. Dark blue
Answer and explanation

D: Dark blue

Chlorine oxidises iodide to iodine: Cl₂ + 2I⁻ → I₂ + 2Cl⁻. Liberated iodine forms a dark-blue complex with starch, giving the characteristic initial colour.

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Question 38

Converting molten iron to steel in the basic oxygen process chiefly removes impurities by

  1. Treatment with acids
  2. Oxidation
  3. Blast reduction
  4. Treatment with alkalis
Answer and explanation

B: Oxidation

In basic oxygen steelmaking, oxygen oxidises excess carbon and other impurities in molten iron. Carbon leaves mainly as gases, while suitable oxides enter the slag.

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Question 40

Ethene reacts with hydrogen bromide to form

  1. CH₂Br₂
  2. CH₃CH₂Br
  3. C₂H₂Br₂
  4. CHBr₃
Answer and explanation

B: CH₃CH₂Br

Hydrogen bromide adds across ethene’s double bond: CH₂=CH₂ + HBr → CH₃CH₂Br. Both ethene carbons are equivalent, so no regioselectivity distinction is needed.

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Question 41

For a simple monosaccharide represented by (CH₂O)ₙ, what is the C:H:O atom ratio?

  1. 3:1:1
  2. 2:1:1
  3. 1:2:1
  4. 1:1:1
Answer and explanation

C: 1:2:1

Simple monosaccharides such as glucose have the general formula (CH₂O)ₙ, giving C:H:O = 1:2:1. This ratio is not universal for every carbohydrate; polysaccharide formation removes water.

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Question 44

The condensed structural formula of ethyl butanoate is

  1. C₃H₇COOC₂H₅
  2. C₂H₅COOC₃H₇
  3. C₄H₉COOC₂H₅
  4. C₂H₅COOC₄H₉
Answer and explanation

A: C₃H₇COOC₂H₅

Butanoate contributes CH₃CH₂CH₂COO–, written C₃H₇COO–. The ethyl group attached through oxygen is C₂H₅, so ethyl butanoate is C₃H₇COOC₂H₅.

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Question 45

Which reaction type is characteristic of benzene?

  1. Addition
  2. Hydrolysis
  3. Polymerisation
  4. Substitution
Answer and explanation

D: Substitution

Benzene characteristically undergoes electrophilic substitution, replacing a ring hydrogen while restoring its aromatic electron system. Addition is possible under more forcing conditions but is not its characteristic reaction.

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Question 46

On heating under reflux with excess acidified K₂Cr₂O₇, ethanol forms

  1. Ethanedioic acid
  2. Ethanol
  3. Ethyl ethanoate
  4. Ethanoic acid
Answer and explanation

D: Ethanoic acid

Excess acidified dichromate oxidises ethanol through ethanal to ethanoic acid. Heating under reflux retains volatile material and allows oxidation to the carboxylic acid.

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Question 48

In petroleum refining, the process that rearranges hydrocarbon structures to improve fuel quality is

  1. Catalytic cracking
  2. Hydrocracking
  3. Polymerisation
  4. Reforming
Answer and explanation

D: Reforming

Catalytic reforming rearranges hydrocarbon structures and can form branched, cyclic or aromatic molecules. It improves fuel properties such as octane rating; cracking chiefly breaks larger molecules into smaller ones.

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Question 49

The main structural substance in cotton fibre is

  1. Starch
  2. Cellulose
  3. Fat
  4. Oil
Answer and explanation

B: Cellulose

Cotton fibres consist mainly of cellulose, a polymer of glucose units joined by β(1→4) glycosidic bonds. Starch is a different glucose polymer used chiefly for energy storage.

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Question 50

The principal hydrocarbon constituent of natural gas is

  1. Methane
  2. Ethane
  3. Propane
  4. Butane
Answer and explanation

A: Methane

Natural gas consists mainly of methane, CH₄, although its composition varies and it can also contain ethane, propane and other components.

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