JAMB Chemistry 1994

22 reviewed questions with answers and explanations.

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Question 4

8 g of CH₄ occupies 11.2 dm³ at s.t.p. What volume would 22 g of CH₃CH₂CH₃ occupy under the same conditions? [C = 12, H = 1]

  1. 3.7 dm³
  2. 11.2 dm³
  3. 22.4 dm³
  4. 33.6 dm³
Answer and explanation

B: 11.2 dm³

CH₄ has molar mass 16 g mol⁻¹, so 8 g contains 0.5 mol. CH₃CH₂CH₃ has molar mass 44 g mol⁻¹, so 22 g also contains 0.5 mol. Equal amounts of gas occupy equal volumes at the same temperature and pressure: 11.2 dm³.

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Question 5

To what temperature must a fixed amount of gas initially at 273 K be heated to double both its volume and pressure?

  1. 298 K
  2. 546 K
  3. 819 K
  4. 1092 K
Answer and explanation

D: 1092 K

For a fixed amount of gas, PV/T is constant. Doubling both pressure and volume multiplies PV by four, so the absolute temperature must become 4 × 273 = 1092 K.

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Question 6

A gas has relative molecular mass 2Y. Using the hydrogen-reference convention for vapour density, what is Y?

  1. The mass of the gas
  2. The vapour density of the gas
  3. The volume of the gas
  4. The temperature of the gas
Answer and explanation

B: The vapour density of the gas

Vapour density relative to hydrogen compares the masses of equal gas volumes at the same temperature and pressure. Since hydrogen has relative molecular mass 2, Mr = 2 × vapour density. Thus Y is the vapour density.

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Question 7

At the same temperature and pressure, gases X and Y have densities 0.5 g dm⁻³ and 2.0 g dm⁻³ respectively. What is the rate of diffusion of X relative to Y?

  1. 0.1
  2. 0.5
  3. 2.0
  4. 4.0
Answer and explanation

C: 2.0

By Graham’s law, diffusion rate is inversely proportional to the square root of gas density under the same conditions. Hence rate(X)/rate(Y) = √(2.0/0.5) = √4 = 2.0.

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Question 8

Heating a gas in a fixed-volume container increases its pressure because

  1. It decreases the number of collisions between molecules
  2. The gas molecules bombard the container walls more frequently
  3. It increases the number of collisions between molecules
  4. It causes the molecules to combine
Answer and explanation

B: The gas molecules bombard the container walls more frequently

Heating increases the average kinetic energy of the gas molecules. In a fixed-volume container they strike its walls more frequently and with greater momentum, increasing the pressure.

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Question 11

Which of these properties generally decreases down a group in the periodic table?

  1. Atomic radius
  2. Ionic radius
  3. Electropositivity
  4. Electronegativity
Answer and explanation

D: Electronegativity

Down a group, additional electron shells increase shielding and the distance between the nucleus and bonding electrons. Electronegativity therefore generally decreases. Atomic size and electropositivity generally increase.

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Question 14

Which option gives the numbers of protons, neutrons and electrons, respectively, in a neutral deuterium atom?

  1. 1, 0, 0
  2. 1, 0, 1
  3. 1, 1, 1
  4. 1, 2, 1
Answer and explanation

C: 1, 1, 1

Deuterium is hydrogen-2. A neutral deuterium atom has one proton, one neutron and one electron, giving mass number 2 and atomic number 1.

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Question 15

A stoppered U-tube contains anhydrous calcium chloride. A known volume of air enters through one side, passes through the calcium chloride, and leaves through the other side. This apparatus is useful for determining the amount of

  1. Oxygen in air
  2. Water vapour in air
  3. Carbon dioxide in air
  4. Argon in air
Answer and explanation

B: Water vapour in air

Anhydrous calcium chloride absorbs water vapour. Passing a known volume of air through the tube allows the absorbed water to be determined from its mass gain, so the apparatus measures water vapour in air.

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Question 21

Which of the following compounds is a base?

  1. CO₂
  2. CaO
  3. H₃PO₃
  4. CH₃COOH
Answer and explanation

B: CaO

Calcium oxide is a basic oxide: it neutralises acids to form a salt and water, for example CaO + 2HCl → CaCl₂ + H₂O. CO₂ is an acidic oxide, while H₃PO₃ and CH₃COOH are acids.

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Question 23

What volume of oxygen at s.t.p. is liberated by 9650 C during electrolysis, assuming the charge is used for oxygen evolution? [Molar gas volume = 22.4 dm³ mol⁻¹; F = 96,500 C mol⁻¹]

  1. 22.4 dm³
  2. 11.2 dm³
  3. 1.12 dm³
  4. 0.560 dm³
Answer and explanation

D: 0.560 dm³

Producing one mole of O₂ from water or hydroxide requires four moles of electrons. The charge supplies 9650/96500 = 0.100 mol electrons, giving 0.0250 mol O₂. At s.t.p., its volume is 0.0250 × 22.4 = 0.560 dm³.

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Question 24

In electrolytic purification using a copper(II) chloride solution, crude copper should be

  1. Both the anode and the cathode
  2. The cathode
  3. The anode
  4. Dissolved in the solution
Answer and explanation

C: The anode

The impure copper is the anode, where copper dissolves by oxidation: Cu → Cu²⁺ + 2e⁻. Copper ions are reduced and deposited on a separate cathode. This transfers copper away from the impure electrode.

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Question 25

H⁻ + H₂O(l) → H₂(g) + OH⁻(aq). What can be inferred?

  1. The reaction is a double decomposition
  2. The hydride ion is a reducing agent
  3. The hydride ion is an oxidising agent
  4. The reaction is neutralisation
Answer and explanation

B: The hydride ion is a reducing agent

Hydrogen in H⁻ has oxidation number −1 and becomes 0 in H₂. The hydride ion is oxidised, so it acts as the reducing agent. Hydrogen from water is reduced from +1 to 0.

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Question 28

MnO₄⁻(aq) + 8H⁺(aq) + 5Fe²⁺(aq) → Mn²⁺(aq) + 5Fe³⁺(aq) + 4H₂O(l). How does the oxidation number of manganese change?

  1. +7 to +2
  2. +6 to +2
  3. +5 to +2
  4. +4 to +2
Answer and explanation

A: +7 to +2

In MnO₄⁻, four oxygen atoms contribute −8. The ion has charge −1, so manganese is +7. In Mn²⁺ it is +2, giving a change from +7 to +2.

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Question 30

Which statement is true for an exothermic reaction at equilibrium?

  1. Increasing temperature increases the equilibrium constant
  2. Increasing temperature decreases the equilibrium constant
  3. Adding a catalyst increases the equilibrium constant
  4. Adding a catalyst decreases the equilibrium constant
Answer and explanation

B: Increasing temperature decreases the equilibrium constant

For an exothermic forward reaction, increasing temperature favours the reverse, endothermic direction. The equilibrium constant for the forward reaction decreases. A catalyst changes the rate of reaching equilibrium but not the equilibrium constant.

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Question 31

Which products form when ammonium nitrate crystals are cautiously heated as described in the question?

  1. N₂O and steam
  2. NO₂ and ammonia
  3. N₂O₄ and NO₂
  4. NO and NO₂
Answer and explanation

A: N₂O and steam

The controlled thermal decomposition represented at this level is NH₄NO₃ → N₂O + 2H₂O. Its gaseous products are nitrous oxide and steam.

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Question 36

Which solution gives a white precipitate with barium chloride solution acidified with hydrochloric acid?

  1. Sodium carbonate
  2. Sodium sulfate
  3. Sodium sulfite
  4. Sodium sulfide
Answer and explanation

B: Sodium sulfate

Sulfate ions form white barium sulfate, which remains insoluble in dilute hydrochloric acid: Ba²⁺ + SO₄²⁻ → BaSO₄(s). Acid removes interference from carbonate and sulfite by converting them into gaseous products.

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Question 37

Why is SO₃ not dissolved directly in water during manufacture of H₂SO₄ by the Contact process?

  1. The reaction of SO₃ with water is violently exothermic
  2. Acid is usually added to water, never water to acid
  3. SO₃ does not dissolve readily in water
  4. SO₃ is an acidic gas
Answer and explanation

A: The reaction of SO₃ with water is violently exothermic

The direct reaction of SO₃ with water releases intense heat and produces a fine sulfuric-acid mist that is difficult to collect. In the Contact process, SO₃ is instead absorbed in concentrated sulfuric acid before controlled dilution.

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Question 38

In an electrolytic arrangement used to protect iron from corrosion, the iron is

  1. Made the cathode
  2. Made the anode
  3. Used with a metal of lower electropositive potential
  4. Initially coated with tin
Answer and explanation

A: Made the cathode

Cathodic protection supplies electrons to the iron so that it does not undergo the anodic oxidation Fe → Fe²⁺ + 2e⁻. The protected iron is made the cathode.

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Question 43

When sodium is added to ethanol, the products are

  1. Sodium hydroxide and water
  2. Sodium hydroxide and hydrogen
  3. Sodium ethoxide and water
  4. Sodium ethoxide and hydrogen
Answer and explanation

D: Sodium ethoxide and hydrogen

Sodium replaces the hydroxyl hydrogen of ethanol to form sodium ethoxide and hydrogen gas: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. The oxygen remains part of the ethoxide ion.

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Question 44

The general formula of alkanones is

  1. RCHO
  2. R₂CO
  3. RCOOH
  4. RCOOR
Answer and explanation

B: R₂CO

An alkanone has a carbonyl carbon bonded to two alkyl groups, represented by R₂CO. The two alkyl groups may be the same or different. RCHO represents an aldehyde, RCOOH a carboxylic acid and RCOOR an ester.

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Question 45

When sodium ethanoate is treated with a few drops of concentrated sulfuric acid, one product is

  1. CH₃COOH
  2. CH₃COOCH₃
  3. CH₃COOC₂H₅
  4. C₂H₅COOCH₃
Answer and explanation

A: CH₃COOH

Sulfuric acid protonates the ethanoate ion, releasing ethanoic acid: CH₃COONa + H₂SO₄ → CH₃COOH + NaHSO₄. This is an acid–base reaction.

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Question 49

Which carbon atomic orbitals are hybridised for bonding with hydrogen in methane?

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

C: 2s and 2p

One carbon 2s orbital and three carbon 2p orbitals form four sp³ hybrid orbitals in the bonding model for methane. Each overlaps with a hydrogen 1s orbital to form a C–H sigma bond.

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