All 41 questions from the Joint Admissions and Matriculation Board (JAMB) Chemistry 2021 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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How many isomers does hexane have?
Hexane (C₆H₁₄) has 5 structural isomers: (1) n-hexane; (2) 2-methylpentane; (3) 3-methylpentane; (4) 2,2-dimethylbutane; (5) 2,3-dimethylbutane.
3.56 g of CuSO₄·xH₂O is heated in a crucible until a constant mass of 3.20 g is obtained. Determine the value of x.
Mass of H₂O = 3.56 − 3.20 = 0.36 g. CuSO₄/H₂O mole ratio: (3.20/160) : (0.36/18) = 0.02 : 0.02 = 1:1. Therefore x = 1, formula is CuSO₄·H₂O.
What is the pH of a 0.001 mol dm⁻³ solution of sodium hydroxide?
NaOH → Na⁺ + OH⁻. [OH⁻] = 0.001 = 10⁻³ M. pOH = 3. pH = 14 − pOH = 14 − 3 = 11.
When a few drops of water are added to a sample of anhydrous cobalt(ii) chloride, the colour of the salt becomes
Anhydrous CoCl₂ is blue. On hydration with water: CoCl₂ + 6H₂O → CoCl₂·6H₂O (pink). This colour change is used as a test for water.
The sequence of separation techniques required to separate a mixture of sand, sodium chloride and iodine is
First: sublimation (heat to remove iodine as vapour). Then: dissolve residue in water (dissolution). Then: filter to remove sand. Then: evaporate filtrate to recover NaCl.
Which of the following gases cannot be collected by downward delivery?
Downward delivery collects gases denser than air. NH₃ (M.M. = 17) is less dense than air (M.M. ≈ 29), so it is collected by upward delivery. Cl₂, SO₂ and H₂S are all denser than air.
What is the shape of BF₃?
BF₃: B has 3 bonding pairs and 0 lone pairs. VSEPR gives trigonal planar geometry with F–B–F angles of 120°. Hybridisation is sp².
Which of the following salts is a normal salt?
KCl is a normal salt (all H⁺ replaced). NaHSO₄ is an acid salt (one H⁺ remaining). Mg(OH)Cl is a basic salt (one OH⁻ remaining). KAl(SO₄)₂·12H₂O is a double salt.
Which of the following is true about the bleaching action of chlorine and sulphur(iv) oxide?
Cl₂ bleaches by oxidation (oxidises coloured matter). SO₂ bleaches by reduction. Both require water. Cl₂ bleaching is permanent; SO₂ bleaching can be reversed by atmospheric oxidation.
The process by which a salt loses its water of crystallisation on exposure to the atmosphere is known as
Efflorescence: loss of water of crystallisation in air (e.g. Na₂CO₃·10H₂O → Na₂CO₃·H₂O). Deliquescence: absorbing moisture until dissolving. Hygroscopy: absorbing moisture without dissolving.
The general formula of alkanones is
R₂CO = alkanone (ketone). RCHO = alkanal (aldehyde). RCOOR = alkanoate (ester). RCONH₂ = alkanamide (amide).
The electronic configuration of an element is 1s²2s²2p⁶3s²3p⁵. It can be deduced that the element (i) is a non-metal (ii) it is a metal (iii) is a p-block element (iv) is a transition element (v) has five electrons in its outermost shell
Configuration = Cl (chlorine). Last orbital is 3p → p-block element. Chlorine is a non-metal. Outermost shell (n=3): 3s²3p⁵ = 7 electrons total in outer shell. Not a transition element (no d orbitals).
Arrange the following in decreasing order of ionisation energy: Be, N, C, Li
Across period 2: Li < Be < B < C < N (with slight anomaly: N > O due to half-filled stability). Decreasing IE: N > C > Be > Li.
The property harnessed in separating air into its components is
Air is liquefied and then fractionally distilled. N₂ (b.p. −196°C) and O₂ (b.p. −183°C) are separated based on their different boiling points.
A gas occupies a volume of 500 cm³ at 30°C. At what temperature will its volume become double this initial value, provided that the pressure is kept constant?
Charles' Law: V₁/T₁ = V₂/T₂. T₁ = 303 K, V₂ = 1000 cm³. T₂ = (1000 × 303)/500 = 606 K = 333°C.
For a reaction to be spontaneous, which of the following is required?
ΔG = ΔH − TΔS. For spontaneity, ΔG < 0. This requires TΔS > ΔH (so that ΔH − TΔS < 0).
2SO₂(g) + O₂(g) → 2SO₃(g), ΔH = −ve. The yield of SO₃ can be improved by
Reaction has fewer moles of gas on right (2 vs 3). By Le Chatelier's principle, increasing pressure favours the side with fewer moles of gas (right), increasing SO₃ yield. Temperature increase would decrease yield (exothermic reaction).
The catalyst that is used to speed up the rate of the thermal decomposition of KClO₃ is
2KClO₃ →(MnO₂, Δ)→ 2KCl + 3O₂. MnO₂ (manganese dioxide) catalyses this decomposition reaction. V₂O₅ catalyses SO₂ oxidation in the Contact Process.
The alkanol produced as a by-product of saponification (soap making) is
Saponification: fat (glyceryl ester) + NaOH → soap (sodium fatty acid salt) + glycerol (propane-1,2,3-triol). Glycerol is the by-product.
Which of the following hydrocarbons are capable of undergoing substitution reactions?
Alkanes undergo free radical substitution (e.g. halogenation). C₂₀H₄₂ is a large alkane. Unsaturated hydrocarbons (alkenes) normally undergo addition reactions, not substitution.
Which of the following is not redox?
A redox reaction requires BOTH oxidation AND reduction. Fe²⁺ → Fe³⁺ + e⁻ shows only oxidation (Fe²⁺ is oxidised to Fe³⁺). There is no corresponding reduction half-reaction shown.
What mass of silver will be deposited at the cathode when a current of 5 A is passed through a solution of AgNO₃ for 1 hour, 1 minute and 1 second? [Ag = 108, 1F = 96500 C]
t = 1×3600 + 1×60 + 1 = 3661 s. Q = It = 5 × 3661 = 18305 C. m = (M × Q)/(n × F) = (108 × 18305)/(1 × 96500) = 20.5 g.
Bronze is an alloy of
Bronze = copper + tin. Brass = copper + zinc. Cupronickel = copper + nickel. Steel = iron + carbon.
Which is the petroleum fraction used in jet engines?
Kerosene (aviation fuel/jet fuel) is the petroleum fraction used in jet engines. Diesel is used in diesel engines; petrol in petrol engines; lubricating oil reduces friction.
The three isotopes of hydrogen are ¹H, ²H and ³H. The heaviest isotope of hydrogen is
Tritium (³H): proton = 1, neutron = 2, mass number = 3. Deuterium (²H): mass number = 2. Protium (¹H): mass number = 1. Tritium is the heaviest.
In ¹H (protium), if an electron is lost, only one proton remains. Why is the hydrogen ion regarded as a proton?
¹H (protium) has 1 proton and 1 electron. When it loses its one electron (ionisation), only the proton remains. H⁺ = proton. This is why the hydrogen ion is regarded as simply a proton.
The number of hydroxonium ions produced by one molecule of an acid is its basicity. The respective basicities of H₂SO₄, HNO₃, HCl and H₃PO₄ are
H₂SO₄ = 2 (diprotic), HNO₃ = 1 (monoprotic), HCl = 1 (monoprotic), H₃PO₄ = 3 (triprotic). Basicities = 2, 1, 1, 3.
When gases are mixed, the chance of finding a molecule of any of those gases at its original position drastically reduces. This is an example of increase in
Entropy (S) is a measure of disorder/randomness. When gases mix, disorder increases (more possible positions for each molecule), so entropy increases.
The paper will turn black due to the formation of lead(ii) sulphide. A piece of filter paper moistened with lead(ii) ethanoate turns black when dropped into which gas?
H₂S + Pb(CH₃COO)₂ → PbS↓ (black) + 2CH₃COOH. The black precipitate of lead(ii) sulphide forms when lead ethanoate paper contacts hydrogen sulphide gas.
The two factors to be considered before choosing a method for collecting a gas are
Two factors for gas collection: (i) Solubility — a gas that is very soluble in water cannot be collected over water (e.g. NH₃, HCl, SO₂); (ii) Density — determines upward or downward delivery.
The NaOH + NH₄Cl → NaCl + H₂O + Y reaction. Ammonia is produced in the laboratory when a base is heated with an ammonium salt. What is Y?
NaOH + NH₄Cl → NaCl + H₂O + NH₃. The base (NaOH) reacts with the ammonium salt (NH₄Cl) when heated to produce ammonia (NH₃).
The carbon atoms in ethane are of what hybridisation type?
Ethane (CH₃–CH₃) has only C–C and C–H single bonds. Each carbon has 4 single bonds and 0 π bonds → sp³ hybridisation with tetrahedral geometry.
Which of the following constitutes of air acts as a diluent?
Nitrogen makes up ~78% of air and acts as a diluent, reducing the concentration of oxygen so that combustion is not too rapid/explosive.
Which of the gases is employed as anaesthesia?
Nitrous oxide (N₂O) is known as 'laughing gas' and is used as a mild anaesthetic and analgesic, particularly in dentistry and minor surgery.
A metal X forms soluble trioxosulphate(iv) salt. The metal X is
Generally, all sodium, potassium and ammonium salts are soluble. Potassium sulphite (K₂SO₃) is soluble. BaSO₃ is insoluble; Al₂(SO₃)₃ hydrolyses; MnSO₃ is insoluble.
Which of the following statements is correct? (a) Atomic size decreases down the group (b) Cations are smaller than the parent atoms (c) Anions are larger than the parent atoms (d) Atomic size increases across the period
Anions gain electrons → increased electron-electron repulsion → larger radius than parent atom. Cations lose electrons → smaller radius. Atomic size increases DOWN the group (more shells). Atomic size decreases ACROSS the period.
At room temperature, fluorine is a gas, chlorine is also gaseous, bromine is a liquid while iodine is solid. Which of the following halogens is liquid at room temperature?
At room temperature (~25°C): F₂ and Cl₂ are gases; Br₂ is a liquid; I₂ is a solid. Bromine is the only halogen that is liquid at standard room temperature.
The diatomic elements are nitrogen, oxygen, fluorine, hydrogen, chlorine, bromine and iodine. Which of the following elements is diatomic?
The diatomic elements are H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂. Oxygen (O₂) is diatomic. Noble gases (Ne, Ar) are monoatomic; metals like Fe and Na are not diatomic.
C₃H₄Cl₂ has four isomers: 1,1-dichloropropane; 2,2-dichloropropane; 1,2-dichloropropane; and 1,3-dichloropropane. How many isomers does C₃H₆Cl₂ have?
C₃H₆Cl₂ has 4 structural isomers: 1,1-dichloropropane; 2,2-dichloropropane; 1,2-dichloropropane; 1,3-dichloropropane.
The main function of limestone in the blast furnace is to
Limestone (CaCO₃) decomposes to CaO in the blast furnace: CaCO₃ → CaO + CO₂. CaO then reacts with silica impurities (SiO₂) to form slag (CaSiO₃), removing impurities.
Which of the following statements about thermoplastic material is correct?
Thermoplastics soften and melt reversibly on heating. Thermosets do not soften — they decompose or char. Examples of thermoplastics: PVC, polyethene, perspex.
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