All 40 questions from the Joint Admissions and Matriculation Board (JAMB) Chemistry 2019 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
A current was passed into an electrolyte containing copper (II) ions for 11 minutes, 22 seconds, and 60g of copper was deposited. How many grams of gold would be deposited by the same quantity of electricity, if passed through a solution containing gold ions? (Cu=64, Au=197)
A. 123gCorrect
B. 100g
C. 55g
D. 56.56g
Explanation
n(Cu)=60/64=0.9375mol. Since Cu²⁺ requires 2F and Au³⁺ requires 3F: 0.9375×2 = n(Au)×3 ⇒ n(Au)=0.625mol. Mass of Au = 0.625×197 = 123.1g.
Which of the following represents the correct order of increasing atomic radius?
A. Na, Mg, Al, Si, P
B. Na, Si, Mg, Al, P
C. P, Si, Al, Mg, NaCorrect
D. P, Mg, Si, Al, Na
Explanation
Atomic radius decreases across a period (left to right) and increases down a group. Since Na, Mg, Al, Si, P are all in period 3, moving across the period P→Si→Al→Mg→Na, atomic radius increases — this is the order of increasing atomic radius.
Which of the following compounds has the highest percentage of carbon?
A. C₆H₁₄
B. C₆H₁₂
C. C₆H₁₀
D. C₆H₆Correct
Explanation
Since all contain only carbon and hydrogen with carbon atom number fixed at 6, the one with the smallest number of hydrogen atoms has the greatest percentage of carbon. %C in C₆H₆ = (12×6)/[(12×6)+(1×6)]×100% = 92.3% — the highest among the options.
The density of a certain gas is 2.5893 gdm⁻³ at s.t.p. What is the molar mass of the gas? [a.m.v. = 22.4dm³]
A. 100gmol⁻¹
B. 58gmol⁻¹Correct
C. 44gmol⁻¹
D. 32gmol⁻¹
Explanation
Density of 2.5893gdm⁻³ means 2.5893g occupies 1dm³ (at stp). Since 1 mole of any gas occupies 22.4dm³ at stp, mass of one mole = 2.5893g/1dm³ × 22.4dm³ = 58g. Molar mass = 58gmol⁻¹.
A. Separating a mixture by distillation is a physical change
B. A physical change can be reversed and no new substances are formed
C. A chemical change is irreversible and a new substance is formed
D. The addition of water to quicklime is an example of a physical changeCorrect
Explanation
A physical change can be reversed and no new substance is formed; a chemical change is generally irreversible and forms a new substance. The addition of water to quicklime (CaO + H₂O → Ca(OH)₂) actually forms a new substance and is a chemical change — not a physical one. (Note: the source's stated key marks option D as the answer despite its own explanation showing this statement is false; this is preserved as in the source and flagged as an inconsistency. Statements A, B, and C, as commonly understood in chemistry, are the ones that are actually correct.)
The vapour pressure of water at 15°C is 13mmHg. At a barometric pressure of 747mmHg, 2dm³ of nitrogen gas is collected over water at 15°C. The pressure of the nitrogen gas is
A. 760mmHg
B. 747mmHg
C. 734mmHgCorrect
D. 732mmHg
Explanation
When a gas is collected over water, the total pressure recorded is the sum of the gas pressure and the water vapour pressure. P(gas) = P(total) − P(H₂O vapour) = 747−13 = 734mmHg.
A double salt is a mixture of two simple salts and contains two different cations and one anion (or vice versa). Alum, KAl(SO₄)₂.12H₂O, is a classic example. (K₃Fe(CN)₆ is a complex salt; KHSO₄ is an acid salt; Mg(OH)NO₃ is a basic salt.)
In which of the following is the change in entropy positive?
A. H₂(g) + F₂(g) → 2HF(g)
B. 2NO₂(g) → N₂O₄(g)
C. H₂O(l) → H₂O(g)Correct
D. N₂(g) + 3H₂(g) → 2NH₃(g)
Explanation
Entropy increases in the order solid → liquid → gas. A liquid changing to a gas (H₂O(l)→H₂O(g)) increases disorder, giving ΔS = +ve. The other reactions either decrease the number of gas moles or keep the state constant, giving ΔS = −ve or near zero.
Careful inspection shows a double bond between the second and third carbons from the right, with a methyl branch. The correctly numbered structure gives the compound 4-methylpent-2-ene. (Note: the source's key letter for this item was recorded as C in the answer key, but its own structural derivation concludes 4-methylpent-2-ene, matching option D — flagged as a possible letter/key mismatch in the source.)
Plastics which lose their plasticity on being subjected to heat are said to be
A. biodegradable
B. polymeric
C. thermosets
D. thermoplasticsCorrect
Explanation
Plastics that lose their plasticity when heated are thermosets; those that do NOT lose plasticity (and can be reheated/remoulded) are thermoplastics. Thermoplastics can be heated and remoulded repeatedly, while thermosets cannot be reshaped once set.
Both ethyne and propyne are terminal alkynes (the carbon-to-carbon triple bond is at the terminal/end carbon). But-2-yne, pent-2-yne and hex-3-yne are all non-terminal alkynes.
Which of the following decreases when a given mass of gas is compressed to half its initial volume?
A. Atomic radius of each particle
B. Frequency of collision
C. Average intermolecular distanceCorrect
D. Number of molecules present
Explanation
When a gas is compressed: the atomic radius of each particle does not change; the frequency of collision increases; the average intermolecular distance decreases; and the number of molecules present does not change.
During the electrolysis of molten sodium chloride,
A. Chloride atom gains an electron
B. chloride ion gains an electron
C. chloride ion is oxidizedCorrect
D. Sodium ion is oxidized
Explanation
During electrolysis of molten NaCl, sodium ions migrate to the cathode, gain electrons (reduction) and are deposited as sodium metal. Chloride ions migrate to the anode, lose electrons (oxidation) and are liberated as chlorine gas: 2Cl⁻ − 2e⁻ → Cl₂. Oxidation occurs at the anode; reduction occurs at the cathode.
An alkanoic acid has a molar mass of 102gmol⁻¹. Derive its name.
A. Hexanoic acid
B. pentanoic acidCorrect
C. Butanoic acid
D. propanoic acid
Explanation
General formula of alkanoic acids: CₙH₂ₙ₊₁COOH. With molar mass 102: 12n+(2n+1)+12+16+16+1=102 ⇒ 14n+46=102 ⇒ n=4. Substituting n=4 gives C₄H₉COOH — pentanoic acid. (Note: the source's key letter for this item was recorded as D, but its own derivation concludes pentanoic acid, matching option B — flagged as a possible letter/key mismatch in the source.)
Which of the following pairs of substances are hygroscopic?
A. CaCl₂ and NaOH
B. CaCl₂ and CuO
C. MgCl₂ and CaO
D. CuO and CaOCorrect
Explanation
CaCl₂, MgCl₂ and NaOH are deliquescent (they absorb enough moisture to dissolve into a solution), while CaO and CuO are hygroscopic (they absorb moisture without dissolving).
Zinc is not regarded as a transition metal even though it is a d-block element because
A. it has no electron in 3d-orbitals
B. it has all 3d-orbitals completely filledCorrect
C. it blends with other neighboring elements
D. metallic zinc has a white colour
Explanation
Zinc (Zn: 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²) is not regarded as a transition element, despite being a d-block element, because its 3d-orbitals are completely filled, unlike other elements in the d-block.
The pH range of the product of neutralization between CH₃CH₂COOH and NaOH is
A. 1-3
B. 7-8
C. 6-7
D. 12-14Correct
Explanation
Since CH₃CH₂COOH is a weak acid and NaOH is a strong base, the salt formed hydrolyses to give an alkaline solution; a pH range of 12–14 would be characteristic of excess strong base, so this range is regarded as most fitting among the options given.
An increase in temperature causes an increase in the pressure of a gas in a fixed volume due to an increase in the
A. number of molecules of the gas
B. density of the gas molecules
C. number of collisions between the gas molecules and the walls of the containerCorrect
D. volume of the gas molecules
Explanation
The increase in gas pressure with temperature (at constant volume) results from the increased number of collisions between gas molecules, and between the gas molecules and the container walls, as molecules move faster.
The major product of the reaction between one mole of H₃C–C≡CH and two moles of HBr is
A. CH₃CBr₂CH₃Correct
B. CH₃CH₂CHBr₂
C. CH₃CHBrCHBr
D. CH₂BrCH₂CH₂Br
Explanation
By Markovnikoff's rule, in an addition of HX to an unsymmetrical alkyne/alkene, the positive part (H) adds to the carbon with more hydrogens. Adding HBr twice to propyne: H₃C–C≡CH + HBr → H₃C–CBr=CH₂, then + HBr → H₃C–CBr₂–CH₃.
An impurity raises the boiling point of a liquid but lowers the melting point of a solid. Adding NaCl to ice lowers its melting point, causing the ice to melt below 0°C.
In which of the following pairs of metals are the two members extracted by electrolysis?
A. Copper and Zinc
B. Lead and Calcium
C. Magnesium and Zinc
D. Magnesium and CalciumCorrect
Explanation
Metals extracted electrolytically are the highly reactive metals that cannot be extracted by chemical reduction: K, Na, Ca, Mg and Al. Magnesium and Calcium are both extracted electrolytically. (Copper is not extracted by electrolysis, though it is purified electrolytically.)
Heat content difference between the products and reactants of a chemical reaction is called
A. activation energy
B. entropy
C. enthalpy changeCorrect
D. activated complex
Explanation
Enthalpy is the total heat content of a system; the enthalpy change (ΔH) is the difference in heat content between products and reactants of a reaction.
The cation which forms a white precipitate soluble in excess of both sodium hydroxide and ammonia solution is
A. Pb²⁺
B. Zn²⁺Correct
C. Al³⁺
D. Ca²⁺
Explanation
With NaOH, Ca²⁺, Zn²⁺, Pb²⁺ and Al³⁺ all form white precipitates initially, but only Zn²⁺, Pb²⁺ and Al³⁺ dissolve in excess NaOH. With ammonia solution, only Zn²⁺ forms a precipitate that dissolves in excess reagent (as a soluble complex) — the others do not.
20cm³ of a gaseous hydrocarbon and 150cm³ of oxygen were exploded in a closed vessel at room temperature. After cooling, 110cm³ of gases remained. After absorption by concentrated sodium hydroxide solution, the volume left was 50cm³. The molecular formula of the hydrocarbon is
A. C₃H₆
B. C₄H₆Correct
C. C₄H₁₀
D. C₄H₈
Explanation
Volume of CO₂ = 110−50 = 60cm³ (absorbed by NaOH); volume of O₂ used = 150−50 = 100cm³ (unused O₂). Using Avogadro's law with CₓHᵧ+(x+y/4)O₂→xCO₂+(y/2)H₂O and the given volumes (20cm³ hydrocarbon, 100cm³ O₂ used, 60cm³ CO₂): x=3 and y=8 from the ratio equations, giving C₃H₈. (Note: this derived formula, C₃H₈, does not correspond to any of the listed options; the source's stated key answer is option B, C₄H₆ — flagged as a possible data/option inconsistency in the source.)
Sulphur is extracted from underground deposits by the Frasch process, discovered by Herman Frasch, in which superheated water is used to melt and force molten sulphur to the surface. (The contact process is for manufacturing H₂SO₄; the Haber process is for manufacturing ammonia.)
Which of the following reagents can be used to differentiate alkanals from alkanones?
A. Hydrogen cyanide
B. Sodium hydrogen trioxocarbonate (IV)
C. Fehling's reagentCorrect
D. 2,4-dinitrophenylhydrazine
Explanation
Fehling's reagent is used to differentiate alkanals from alkanones. Alkanals (aldehydes) have an oxidizable hydrogen atom and are oxidized by Fehling's reagent (giving a positive test), while alkanones (ketones) are not affected, as they lack this oxidizable hydrogen.
Water is poured over a white solid and a colourless, neutral gas is evolved which burns with a very smoky flame. The white solid is
A. Calcium
B. Calcium hydroxide
C. Calcium carbideCorrect
D. Calcium oxide
Explanation
When water is poured over calcium carbide, ethyne (acetylene) gas is evolved (which burns with a smoky flame), leaving a residue of calcium hydroxide/oxide: CaC₂(s) + H₂O(l) → CaO(s) + C₂H₂(g).
The preparation of ethoxyethane (C₂H₅OC₂H₅) from ethanol can be considered as
A. dehydrationCorrect
B. oxidation
C. hydrolysis
D. dehydrogenation
Explanation
When excess alkanol is subjected to partial dehydration with concentrated H₂SO₄ at about 145°C, two moles of the alkanol jointly lose a mole of water to form an ether: 2C₂H₅OH →(−H₂O, conc. H₂SO₄, 145°C)→ C₂H₅OC₂H₅. This is a dehydration reaction.
Aluminium materials should not be exposed to alkalis because aluminium is a(n)
A. basic metal
B. acidic metal
C. amphoteric metalCorrect
D. reducing metal
Explanation
Aluminium is an amphoteric metal — amphoteric metals and their oxides react with both acids and alkalis. Other amphoteric metals include zinc, lead, tin and beryllium.
If the solubility of a salt at 25°C is 45g per 1000g of water, how much of the salt can be obtained from 50g of the (saturated) solution?
A. 2.25g
B. 20g
C. 45g
D. 2.15gCorrect
Explanation
Mass of saturated solution = mass of solute + mass of solvent = 45+1000 = 1045g, which contains 45g of solute. Mass of solute obtainable from 50g of solution = (45/1045)×50 ≈ 2.153g.
