JAMB Chemistry 1993

31 reviewed questions with answers and explanations.

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Question 2

Which listed substance is a mixture?

  1. Sulfur powder
  2. Bronze
  3. Distilled water
  4. Ethanol
Answer and explanation

B: Bronze

Bronze is an alloy containing copper and tin, often with smaller amounts of other elements. Its components are mixed rather than combined in one fixed molecular ratio, so it is a mixture. Pure sulfur, distilled water and ethanol are single substances.

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Question 3

How many moles of O₂ are formed by complete decomposition of 2.5 mol of KClO₃?

  1. 2.50 mol
  2. 3.50 mol
  3. 3.75 mol
  4. 7.50 mol
Answer and explanation

C: 3.75 mol

The balanced equation is 2KClO₃ → 2KCl + 3O₂. Two moles of chlorate produce three moles of oxygen, so 2.5 mol produces 2.5×3/2 = 3.75 mol of O₂.

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Question 5

A gas occupies 1.50 dm³ at 25 °C and 1 atm. What volume does the same amount occupy at 100 °C and 1 atm?

  1. 1.88 dm³
  2. 6.00 dm³
  3. 18.80 dm³
  4. 60.00 dm³
Answer and explanation

A: 1.88 dm³

At constant pressure, volume is proportional to absolute temperature. Convert 25 °C and 100 °C to about 298 K and 373 K. Then V₂ = 1.50×373/298 ≈ 1.88 dm³. Celsius temperatures cannot be used directly in the ratio.

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Question 9

An element E has the electronic configuration 1s² 2s² 2p⁶ 3s² 3p³. Reaction of E with a halogen X can give

  1. EX₃ and EX₅
  2. EX₃ only
  3. EX₅ only
  4. EX₂ and EX₃
Answer and explanation

A: EX₃ and EX₅

The configuration contains 15 electrons, identifying phosphorus. Phosphorus can form trihalides and pentahalides, for example PCl₃ and PCl₅; the possible formulae are therefore EX₃ and EX₅.

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Question 10

Two atoms represented as ²³⁵₉₂U and ²³⁸₉₂U are

  1. isomers
  2. allotropes
  3. isotopes
  4. anomers
Answer and explanation

C: isotopes

Both uranium atoms have atomic number 92, so each has 92 protons. Their mass numbers differ: 235 − 92 = 143 neutrons and 238 − 92 = 146 neutrons. Atoms of the same element with different neutron numbers are isotopes.

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Question 11

As the difference in electronegativity between bonded atoms increases, the polarity of the bond

  1. decreases
  2. increases
  3. remains unchanged
  4. reduces to zero.
Answer and explanation

B: increases

A greater electronegativity difference makes the bonding electrons more unevenly shared. The partial charges and bond polarity therefore increase.

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Question 13

Water has a rather high boiling point despite its low molecular mass because of the presence of

  1. hydrogen bonding
  2. covalent bonding
  3. ionic bonding
  4. metallic bonding
Answer and explanation

A: hydrogen bonding

Hydrogen bonds attract neighbouring water molecules to one another. Boiling requires overcoming these intermolecular attractions, giving water a relatively high boiling point for its small molecular mass.

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Question 16

Water can be identified by the use of

  1. Anhydrous copper(II) tetraoxosulfate(VI)
  2. Anhydrous sodium trioxocarbonate(IV)
  3. Potassium heptaoxochromate(VII)
  4. Copper(II) trioxocarbonate(IV)
Answer and explanation

A: Anhydrous copper(II) tetraoxosulfate(VI)

Anhydrous copper(II) sulfate is white and becomes blue when hydrated. This colour change provides a test for water.

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Question 17

Sodium carbonate decahydrate loses some of its water of crystallisation on exposure to the atmosphere. What is this phenomenon called?

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

D: Efflorescence

Efflorescence is the loss of water of crystallisation from a hydrated salt on exposure to air. Sodium carbonate decahydrate is a familiar example.

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Question 19

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Which listed salt has the most rapid increase of solubility with temperature over its plotted range?

  1. CaSO₄
  2. KNO₃
  3. NaCl
  4. KCl

Question 20

NH₃ + H₃O⁺ → NH₄⁺ + H₂O. What can be deduced from this reaction?

  1. a redox reaction has occurred
  2. H3O+ acts as an oxidizing agent
  3. H3O+ acts as an acid
  4. Water acts as an acid
Answer and explanation

C: H3O+ acts as an acid

H₃O⁺ donates a proton to NH₃, forming NH₄⁺ and H₂O. A Brønsted–Lowry acid is a proton donor, so hydronium acts as the acid in this reaction.

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Question 21

What is the molar concentration of a solution containing 4.0 g of sodium hydroxide in 250 cm³ of solution?

  1. 0.40 mol dm⁻³
  2. 0.10 mol dm⁻³
  3. 0.04 mol dm⁻³
  4. 0.02 mol dm⁻³
Answer and explanation

A: 0.40 mol dm⁻³

The molar mass of NaOH is 23 + 16 + 1 = 40 g mol⁻¹. Thus 4.0 g is 0.10 mol. The solution volume is 250 cm³ = 0.250 dm³, giving 0.10/0.250 = 0.40 mol dm⁻³.

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Question 24

Standard reduction potentials are: Cu²⁺(aq) + 2e⁻ → Cu(s), +0.34 V; Fe²⁺(aq) + 2e⁻ → Fe(s), −0.44 V; Ba²⁺(aq) + 2e⁻ → Ba(s), −2.90 V; Zn²⁺(aq) + 2e⁻ → Zn(s), −0.76 V. Which metal is the most powerful reducing agent?

  1. Cu
  2. Fe
  3. Ba
  4. Zn
Answer and explanation

C: Ba

The more negative the standard reduction potential, the greater the tendency of the metal to be oxidised in the reverse process. Barium has the most negative value, −2.90 V, so it is the strongest reducing agent in this comparison.

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Question 25

The oxidation states of chlorine in HOCl, HClO₃ and HClO₄ are respectively

  1. −1, +5 and +7
  2. −1, −5 and +7
  3. +1, +3 and +4
  4. +1, +5 and +7
Answer and explanation

D: +1, +5 and +7

Take hydrogen as +1 and oxygen as −2. Each neutral molecule has total oxidation number zero: chlorine is +1 in HOCl, +5 in HClO₃ and +7 in HClO₄.

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Question 28

The standard enthalpies of formation of CO₂(g), H₂O(g) and CO(g) are −394, −242 and −110 kJ mol⁻¹ respectively. What is the standard enthalpy change for CO(g) + H₂O(g) → CO₂(g) + H₂(g)?

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

A: −42 kJ mol⁻¹

Use ΔH° = sum of product formation enthalpies minus sum of reactant formation enthalpies. H₂(g) has zero standard formation enthalpy, so ΔH° = (−394 + 0) − (−110 − 242) = −42 kJ mol⁻¹.

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Question 29

10 g of a solid is in equilibrium with its own vapour. At the same temperature, another 1 g of the solid is added. Its equilibrium vapour pressure will

  1. Remain the same
  2. Drop
  3. Increase by 1%
  4. Increase by 99%
Answer and explanation

A: Remain the same

At a fixed temperature, the equilibrium vapour pressure of a pure solid is independent of how much solid is present, provided some solid remains. Adding 1 g therefore leaves the equilibrium vapour pressure unchanged.

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Question 30

Two energy profiles, X and Y, describe the same homogeneous gaseous reaction. Energy is plotted against reaction progress. Both profiles start at the same reactant energy and end at the same, lower product energy. Y has a lower peak than X. Which change produces profile Y?

  1. Increase in temperature
  2. Increase in the concentration of a reactant
  3. Addition of a catalyst
  4. Increase in pressure
Answer and explanation

C: Addition of a catalyst

A catalyst provides a reaction pathway with a lower activation-energy barrier. It does not change the energies of the reactants and products, so the reaction enthalpy remains the same.

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Question 31

NaCl(s) + H₂SO₄(l) → HCl(g) + NaHSO₄(s). In this reaction, H₂SO₄ behaves as

  1. A strong acid
  2. An oxidising agent
  3. A good solvent
  4. A dehydrating agent
Answer and explanation

A: A strong acid

Sulfuric acid donates a proton to chloride, producing hydrogen chloride and hydrogensulfate. It is acting as an acid. No water is removed in this equation, and the oxidation states do not change.

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Question 32

Which salt produces its metal, oxygen and nitrogen(IV) oxide on heating?

  1. Silver nitrate
  2. Sodium nitrate
  3. Calcium nitrate
  4. Lithium nitrate
Answer and explanation

A: Silver nitrate

Silver nitrate decomposes to silver, nitrogen dioxide and oxygen: 2AgNO₃ → 2Ag + 2NO₂ + O₂. Sodium nitrate gives a nitrite and oxygen, while the calcium and lithium salts give their metal oxides rather than the metals.

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Question 33

An experiment produces a mixture of CO₂ and CO. Which treatment removes CO₂ while retaining CO?

  1. Pass it over heated copper(II) oxide
  2. Bubble it through concentrated sulfuric acid
  3. Bubble it through sodium hydroxide solution
  4. Bubble it through water
Answer and explanation

C: Bubble it through sodium hydroxide solution

Sodium hydroxide absorbs the acidic oxide CO₂, while CO does not react in this washing step. With sufficient hydroxide, CO₂ + 2NaOH → Na₂CO₃ + H₂O. Passing the mixture over hot CuO would instead consume CO by oxidising it to CO₂.

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Question 34

Which property identifies chloride ions from an ionic chloride in aqueous solution?

  1. They can be decomposed by heat
  2. They react with aqueous AgNO₃ to give a white precipitate soluble in excess ammonia
  3. They explode on contact with dry ammonia gas
  4. They react with concentrated sulfuric acid to give white fumes of chlorine gas
Answer and explanation

B: They react with aqueous AgNO₃ to give a white precipitate soluble in excess ammonia

Chloride ions give a white silver chloride precipitate with aqueous silver nitrate: Ag⁺ + Cl⁻ → AgCl(s). Excess ammonia dissolves it by forming the soluble complex [Ag(NH₃)₂]⁺.

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Question 35

Dilute aqueous solutions of lead(II) nitrate and potassium bromide are mixed and a precipitate forms. Which list gives the resulting solid and dissolved spectator species?

  1. PbO(s), Br⁻(aq) and KNO₃
  2. Br₂, NO₂(g) and PbBr₂(s)
  3. PbO(s), K⁺(aq), Br(aq) and NO₂(g)
  4. PbBr₂(s), K⁺(aq) and NO₃⁻(aq)
Answer and explanation

D: PbBr₂(s), K⁺(aq) and NO₃⁻(aq)

Lead(II) ions and bromide ions form insoluble lead(II) bromide: Pb²⁺ + 2Br⁻ → PbBr₂(s). Potassium and nitrate ions remain in solution as spectator ions. The full reaction is Pb(NO₃)₂ + 2KBr → PbBr₂ + 2KNO₃.

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Question 36

Bronze is an alloy of

  1. Silver and copper
  2. Silver and gold
  3. Copper and nickel
  4. Copper and zinc
  5. Copper and tin
Answer and explanation

E: Copper and tin

Traditional bronze is an alloy of copper and tin. Copper and zinc form brass, which is a different alloy.

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Question 37

Copper reacts with concentrated nitric acid to give which set of products?

  1. Cu(NO₃)₃, NO, N₂O₄ and H₂O
  2. Cu(NO₃)₂, NO and H₂O
  3. CuO, NO₂ and H₂O
  4. Cu(NO₃)₂, NO₂ and H₂O
Answer and explanation

D: Cu(NO₃)₂, NO₂ and H₂O

Concentrated nitric acid oxidises copper to copper(II) nitrate and is reduced mainly to nitrogen dioxide. The balanced equation is Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O.

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Question 39

Al₂O₃(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂O(l). Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2NaAl(OH)₄(aq). These reactions show that Al₂O₃ is

  1. an acidic oxide
  2. an amphoteric oxide
  3. a basic oxide
  4. a neutral oxide
Answer and explanation

B: an amphoteric oxide

Aluminium oxide reacts with sulfuric acid and also with sodium hydroxide. An oxide that reacts with both acids and bases is amphoteric.

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Question 41

The fraction of crude oil used as jet fuel is

  1. refinery gas
  2. diesel oil
  3. kerosene
  4. gasoline
Answer and explanation

C: kerosene

The kerosene fraction is used to produce common aviation turbine fuels such as Jet A and Jet A-1. Refinery gas and gasoline are lighter fractions, while diesel is a heavier fuel fraction.

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Question 42

What is the IUPAC name of CH₃CH(CH₃)CH₂CH(CH₃)CH₂CH₃?

  1. Dimethylhexane
  2. 3,5-dimethylpentane
  3. 1,1-dimethyl-3-methylpentane
  4. 2,4-dimethylhexane
Answer and explanation

D: 2,4-dimethylhexane

The longest carbon chain has six atoms. Numbering from the end nearer the first branch puts methyl groups at positions 2 and 4, rather than 3 and 5. The name is 2,4-dimethylhexane.

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Question 43

Why is tetraethyllead undesirable as an antiknock additive?

  1. It is expensive
  2. It causes pollution through exhaust emissions
  3. It lowers the octane rating of petrol
  4. It is explosive
Answer and explanation

B: It causes pollution through exhaust emissions

Tetraethyllead improves resistance to engine knocking, but burning leaded fuel releases lead-containing pollution in the exhaust. This environmental contamination is the reason for avoiding it as an antiknock additive.

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Question 44

The carbon atoms in ethane are

  1. sp² hybridised
  2. sp³ hybridised
  3. sp²d hybridised
  4. sp hybridised
Answer and explanation

B: sp³ hybridised

Each ethane carbon forms four sigma bonds, three to hydrogen and one to the other carbon. This approximately tetrahedral arrangement corresponds to sp³ hybridisation.

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Question 47

Palm wine turns sour with time because

  1. Its sugar content is converted into alcohol
  2. The carbon dioxide formed during fermentation has a sour taste
  3. It is commonly adulterated by tappers and sellers
  4. Microbial activity produces organic acids in it
Answer and explanation

D: Microbial activity produces organic acids in it

Microorganisms produce organic acids during continued palm-wine fermentation. For example, acetic-acid bacteria can oxidise ethanol to ethanoic acid, increasing acidity and producing a sour taste.

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Question 49

Which reaction represents saponification?

  1. Carboxylic acids reacting with sodium hydroxide
  2. Alkanoate esters reacting with acids
  3. Alkanoate esters reacting with alcohols
  4. Alkanoate esters reacting with sodium hydroxide
Answer and explanation

D: Alkanoate esters reacting with sodium hydroxide

Saponification is alkaline hydrolysis of an ester. An alkanoate ester reacts with sodium hydroxide to form a sodium carboxylate and an alcohol: RCOOR′ + NaOH → RCOONa + R′OH. Fats give soap and glycerol because they are glycerol esters.

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