JAMB Chemistry 1995

26 reviewed questions with answers and explanations.

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Question 1

Chromatography is used to separate components of mixtures which differ in their rates of

  1. Diffusion
  2. Migration
  3. Reaction
  4. Sedimentation
Answer and explanation

B: Migration

Chromatographic components interact differently with the stationary and mobile phases. They therefore travel at different rates, or rates of migration, which separates the mixture.

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Question 2

Which of the following is an example of chemical change?

  1. Dissolving salt in water
  2. Rusting of iron
  3. Melting of ice
  4. Separating a mixture by distillation
Answer and explanation

B: Rusting of iron

Rusting produces new iron-containing oxide and hydroxide substances through reaction with oxygen and water. This is a chemical change. Melting and distillation change physical state or separate substances without this type of chemical transformation.

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Question 3

How many H⁺ ions can 4.9 g of H₂SO₄ provide if both acidic hydrogens are completely released? [S = 32, O = 16, H = 1; Nₐ = 6.02 × 10²³ mol⁻¹]

  1. 3.01 × 10²²
  2. 6.02 × 10²²
  3. 3.01 × 10²³
  4. 6.01 × 10²³
Answer and explanation

B: 6.02 × 10²²

M(H₂SO₄) = 2 + 32 + 4 × 16 = 98 g mol⁻¹. Thus 4.9 g is 0.050 mol acid. Complete release of both acidic hydrogens gives 0.100 mol H⁺, or 0.100 × 6.02 × 10²³ = 6.02 × 10²² ions.

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Question 4

What volume of oxygen remains after complete reaction of 8 cm³ of hydrogen with 20 cm³ of oxygen, with all gas volumes compared at the same temperature and pressure?

  1. 10 cm³
  2. 12 cm³
  3. 14 cm³
  4. 16 cm³
Answer and explanation

D: 16 cm³

The reaction is 2H₂ + O₂ → 2H₂O. At the same temperature and pressure, 8 cm³ of hydrogen consumes 4 cm³ of oxygen. The oxygen left is 20 − 4 = 16 cm³.

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Question 10

A section of the periodic table labels elements by letter. Their atomic numbers are L = 2, G = 3, X = 4, J = 8, E = 9, M = 12, R = 13 and T = 16. Which letter represents a nonmetal that is solid at room temperature?

  1. T
  2. R
  3. J
  4. X
Answer and explanation

A: T

T has atomic number 16, so it represents sulfur, a solid nonmetal at room temperature. R (13) is aluminium, J (8) is oxygen and X (4) is beryllium. The two metals and gaseous oxygen do not satisfy the description.

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Question 11

In the oil-drop experiment, Millikan determined the

  1. Charge-to-mass ratio of the electron
  2. Mass of the electron
  3. Charge of the electron
  4. Mass of the proton
Answer and explanation

C: Charge of the electron

The oil-drop experiment measured electric charges on droplets and showed that they occurred in multiples of a fundamental charge. This determined the magnitude of the electron’s charge.

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Question 12

The stability of ionic solids is generally due to the

  1. negative electron affinity of most atoms
  2. crystal lattice forces
  3. electron pair sharing
  4. positive ionization potentials.
Answer and explanation

B: crystal lattice forces

Oppositely charged ions attract one another throughout an ionic crystal lattice. These electrostatic lattice forces account for much of the stability of ionic solids. Electron-pair sharing instead describes covalent bonding.

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Question 14

Helium is often used in observation balloons because it is

  1. Light and combustible
  2. Light and noncombustible
  3. Heavy and combustible
  4. Heavy and noncombustible
Answer and explanation

B: Light and noncombustible

Helium is less dense than air, providing buoyancy, and it is noncombustible. Its chemically unreactive nature makes it suitable as a lifting gas.

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Question 16

Deliquescent substances are also

  1. Efflorescent
  2. Anhydrous
  3. Hygroscopic
  4. Insoluble
Answer and explanation

C: Hygroscopic

A hygroscopic substance absorbs water from the surrounding air. A deliquescent substance absorbs enough water to dissolve in it, so deliquescence is a special case of hygroscopic behaviour.

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Question 17

Which statement distinguishes colloids from suspensions under ordinary standing conditions? Colloids…

  1. do not scatter light, whereas suspensions do
  2. can be separated by filtration, whereas suspensions cannot
  3. can be separated by a membrane, whereas suspensions cannot
  4. do not settle out on standing, whereas suspensions do
Answer and explanation

D: do not settle out on standing, whereas suspensions do

Colloidal particles are small enough to remain dispersed under ordinary standing conditions. The larger particles of a suspension settle under gravity. Colloids also scatter light, so lack of light scattering is not their defining distinction.

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Question 20

Which aqueous solution is acidic, with pH below 7 at room temperature?

  1. Na₂SO₄(aq)
  2. NaCl(aq)
  3. Na₂CO₃(aq)
  4. NH₄Cl(aq)
Answer and explanation

D: NH₄Cl(aq)

Ammonium ions are weak acids: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺. This makes NH₄Cl solution acidic. Sodium chloride is approximately neutral, while sodium carbonate is basic.

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Question 21

What is the pH of a 2.50 × 10⁻⁵ mol dm⁻³ NaOH solution at 25 °C?

  1. 3.6
  2. 5.0
  3. 9.4
  4. 12.0
Answer and explanation

C: 9.4

NaOH dissociates to give [OH⁻] ≈ 2.50 × 10⁻⁵ mol dm⁻³. Therefore pOH = −log₁₀(2.50 × 10⁻⁵) = 4.60. At 25 °C, pH = 14.00 − 4.60 = 9.40.

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Question 22

In a plot of pH against volume of base added, the pH begins near 6, rises gradually, then rises steeply through a region entirely above pH 7, and finally levels towards a high alkaline pH. Which titration does this pattern represent?

  1. Strong acid versus strong base
  2. Weak acid versus strong base
  3. Strong acid versus weak base
  4. Weak acid versus weak base
Answer and explanation

B: Weak acid versus strong base

The solution begins weakly acidic, and the sharp rise occurs above pH 7. This is the characteristic pattern for titrating a weak acid with a strong base: the conjugate base makes the equivalence mixture alkaline.

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Question 23

In the process of silver-plating a metal M, the metal M is the

  1. Anode, using direct current
  2. Cathode, using alternating current
  3. Anode, using alternating current
  4. Cathode, using direct current
Answer and explanation

D: Cathode, using direct current

Silver ions gain electrons and deposit as metal at the cathode: Ag⁺ + e⁻ → Ag. The object to be plated must therefore be the cathode, and a direct current maintains the required electrode polarity.

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Question 24

How many moles of copper are deposited by 3 faradays of charge through aqueous copper(II) sulfate, assuming complete current efficiency?

  1. 0.5
  2. 1.0
  3. 1.5
  4. 3.0
Answer and explanation

C: 1.5

Copper(II) ions require two electrons per atom: Cu²⁺ + 2e⁻ → Cu. A charge of 3 faradays supplies 3 mol of electrons, so it deposits 3/2 = 1.5 mol of copper.

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Question 25

The anode half-reaction 2Cl⁻(aq) → Cl₂(g) + 2e⁻ is an example of

  1. Ionisation
  2. Oxidation
  3. Reduction
  4. Recombination
Answer and explanation

B: Oxidation

Chloride ions lose electrons in 2Cl⁻ → Cl₂ + 2e⁻. Electron loss is oxidation, and chlorine’s oxidation number increases from −1 to 0.

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Question 27

Cr₂O₇²⁻ + 14H⁺ + 6I⁻ → 2Cr³⁺ + 3I₂ + 7H₂O. What is the change in the oxidation number of oxygen?

  1. 0
  2. 1
  3. 2
  4. 7
Answer and explanation

A: 0

Oxygen has oxidation number −2 both in Cr₂O₇²⁻ and in H₂O. It does not change oxidation state, so the change is zero. Chromium and iodine are the elements undergoing redox changes.

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Question 28

For a reaction at equilibrium with ΔH < 0, which temperature condition favours the forward equilibrium direction?

  1. Low temperature
  2. High temperature
  3. All temperatures
  4. All pressures
Answer and explanation

A: Low temperature

A negative ΔH means the forward reaction releases heat. Lowering temperature favours this exothermic direction, increasing the equilibrium proportion of products. This describes the equilibrium shift, not the speed of reaction.

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Question 33

For N₂(g) + 3H₂(g) → 2NH₃(g), two energy profiles have identical reactant and product energies. Products are lower in energy than reactants. Path I has the higher peak and path II the lower peak. The interval x is the vertical rise from reactant energy to the higher peak; y is the drop from reactant energy to product energy. Comparing these uncatalysed and catalysed paths under the same conditions, which statement about reaction rate is correct?

  1. For path I is higher than for path II
  2. For path II is higher than for path I
  3. Is the same for both paths at all temperatures
  4. Depends on both x and y at all pressures
Answer and explanation

B: For path II is higher than for path I

Path II has a lower activation-energy barrier than path I. For the catalysed pathway of the same reaction under comparable conditions, this allows more successful reaction events and a faster rate. A catalyst leaves the reactant and product energies unchanged.

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Question 34

In the industrial purification of hydrogen made from natural gas, which listed bulk-removal method is used for the accompanying carbon dioxide?

  1. Washing under pressure
  2. Passing the mixture into limewater
  3. Using ammoniacal copper(I) chloride
  4. Drying over phosphorus(V) oxide
Answer and explanation

A: Washing under pressure

Washing the compressed gas with water removes much of the CO₂ because its solubility increases with pressure. Ammoniacal copper(I) chloride is used for CO rather than CO₂; a drying agent removes water.

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Question 37

Which ion forms a white precipitate with aqueous NaOH that dissolves in excess NaOH?

  1. Ca²⁺
  2. Mg²⁺
  3. Zn²⁺
  4. Cu²⁺
Answer and explanation

C: Zn²⁺

Zn²⁺ first gives white Zn(OH)₂ with hydroxide ions. In excess NaOH, the precipitate dissolves to form [Zn(OH)₄]²⁻. Copper(II) hydroxide is blue, while calcium and magnesium hydroxides do not show this amphoteric dissolution.

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Question 38

In iron extraction in the blast furnace, the purpose of adding limestone is to

  1. Release CO₂ for the reaction
  2. Reduce the iron
  3. Increase the strength of iron
  4. Remove impurities
Answer and explanation

D: Remove impurities

Limestone decomposes to CaO. This basic oxide reacts with acidic impurities such as SiO₂ to form calcium silicate slag: CaO + SiO₂ → CaSiO₃. Removing these impurities is the purpose of the limestone flux.

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Question 39

Which compound gives a brick-red colour in a nonluminous Bunsen flame?

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

C: CaCl₂

Calcium compounds produce an orange-red or brick-red flame. Sodium gives yellow, lithium crimson, and magnesium salts give no characteristic visible flame-test colour.

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Question 44

Which of the following pairs has compounds that are isomers?

  1. Propanal and propanone
  2. Ethanoic acid and ethyl methanoate
  3. Ethanoic acid and ethane-1,2-diol
  4. 2-Methylbutane and 2,2-dimethylbutane
Answer and explanation

A: Propanal and propanone

Propanal and propanone both have molecular formula C₃H₆O. They differ in connectivity and functional group: propanal is an aldehyde and propanone a ketone. Each other pair has different molecular formulae.

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Question 49

An example of a secondary amine is

  1. Propylene
  2. Dibutylamine
  3. Methylamine
  4. Trimethylamine
Answer and explanation

B: Dibutylamine

Dibutylamine has two butyl groups bonded to nitrogen and one N–H bond, so it is a secondary amine. Methylamine is primary, trimethylamine is tertiary and propylene is a hydrocarbon.

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Question 50

The relatively high boiling points of alkanols are mainly due to

  1. Ionic bonding
  2. Aromatic character
  3. Covalent bonding
  4. Hydrogen bonding
Answer and explanation

D: Hydrogen bonding

Alcohol molecules form intermolecular hydrogen bonds through their O–H groups. More energy is needed to separate the molecules, giving relatively high boiling points compared with similar-sized molecules without this hydrogen bonding.

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