All 50 questions from the West African Examinations Council (WAEC) Chemistry 2024 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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1. Which of the following gases could be collected by downward displacement of air?
Gases denser than air are collected by upward displacement of air; gases less dense than air (like hydrogen) are collected by downward displacement of air.
2. Which of the following substances has the lowest boiling point?
Ethanol and water have hydrogen bonding, giving them relatively high boiling points. Tetrachloromethane has no hydrogen bonding, so its boiling point is comparatively low.
3. The branch of science that deals with the nature and properties of substances and how one substance can be converted to another is known as
This is the definition of chemistry.
4. The electron configuration 1s²2s²2pₓ² contravenes the
Hund's rule states electrons fill degenerate orbitals singly before pairing; pairing electrons in one 2p orbital while others remain empty violates this rule.
5. The oxidation number of chromium in Na₂Cr₂O₇ is
For Na (+1) and O (-2): 2(+1)+2Cr+7(-2)=0 → 2+2Cr-14=0 → Cr=+6.
6. The most important ore of aluminium is
Bauxite is refined to produce alumina via the Bayer process, then smelted into aluminium using the Hall-Heroult process.
7. Bases normally
Alkaline solutions turn red litmus paper blue.
8. The product of the reaction between ethanol and excess acidified K₂Cr₂O₇ is
Ethanol reacts with excess acidified K₂Cr₂O₇ (an oxidizing agent) to produce ethanoic acid.
9. The product formed when concentrated sodium chloride solution is electrolysed using carbon electrodes is
Electrolysis of concentrated NaCl solution with carbon electrodes produces Cl₂ at the anode, H₂ at the cathode, and a resultant NaOH solution.
10. Which of the following statements about ammonium salt is correct? It
An ammonium salt like NH₄Cl dissolves in water to form a solution whose pH is less than 7.
11. Ethanedioic acid is an organic solid that can be purified by
Oxalic (ethanedioic) acid is purified through crystallization: dissolving in hot water, then cooling, filtering, washing and drying.
12. Which of the following functional groups is present in alkanoic acid?
Alkanoic acids (carboxylic acids) contain the carboxyl group, -COOH.
13. In ethanol, the attractive forces between adjacent molecules are
Both hydrogen bonds and van der Waal's forces act between adjacent ethanol molecules.
14. At 25 °C, the saturated solution of a salt in water was found to contain 0.24 g of the salt in 100 cm³. What is the solubility of the salt in gdm⁻³?
Solubility = mass/volume = 0.24 g / 0.1 dm³ = 2.4 gdm⁻³ (note: printed key marks D; the calculation itself yields 2.4, matching option C — flagged for your review).
15. The reaction represented by the equation Ni²⁺(aq) + Fe(s) → Ni(s) + Fe²⁺(aq) is a redox reaction because
Ni²⁺ is reduced to Ni while Fe is oxidized to Fe²⁺; the species reduced (Ni²⁺) is the oxidizing agent and Fe, which is oxidized, is the reducing agent.
16. Which of the following statements describes transition elements? They
Transition elements have variable oxidation states, form coloured compounds and complex ions, and have high melting/boiling points.
17. Aluminium is suitable for making alloys for aircraft construction because it
Aluminium is lightweight, highly resistant to corrosion, and strong when alloyed with other metals.
18. Which of the following arrangements of elements is in order of increasing ionization energy?
Ionization energy generally increases across a period, giving the expected trend Al < Si < P < S (note: phosphorus's half-filled 3p³ configuration actually makes its real ionization energy exceed sulphur's, but Al<Si<P<S is the intended answer).
19. Consider the reaction C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l). If 0.1 mole of C₃H₈(g) was completely burnt, what volume of CO₂(g) would be produced at stp? [molar volume at stp = 22.4 dm³mol⁻¹]
0.1 mol C₃H₈ produces 0.3 mol CO₂ (1:3 ratio). Volume = 0.3 × 22.4 dm³ = 6.72 dm³.
20. Naturally occurring Boron is made up of 19.9% ¹⁰B and 80.1% ¹¹B. The relative atomic mass of Boron is
R.A.M = (10×19.9/100) + (11×80.1/100) = 1.99 + 8.811 = 10.801 ≈ 10.8.
21. The number of shared pairs of electrons in a molecule of methane is
Methane (CH₄) has four single covalent bonds, each consisting of one shared pair of electrons.
22. Isotopes of the same element have similar chemical properties because they have the same number of
Isotopes have the same number of protons (and electrons), giving them similar chemical properties, despite differing neutron numbers.
23. Which of the following household liquids would be effective in treating someone with too much acid in the stomach?
A substance used to treat excess stomach acidity must be alkaline (pH greater than 7).
24. The percentage by mass of oxygen in MgSO₄.7H₂O is [Mr = 246]
MgSO₄.7H₂O has 11 oxygen atoms. %oxygen = (16×11)/246 × 100% = 176/246 × 100% ≈ 71.5%.
25. Catalysts alter reaction rate by
Catalysts lower the activation energy by providing an alternative reaction pathway (they do not change the enthalpy/energy of reaction).
26. A volume of 400 cm³ of ethane was completely burnt in excess oxygen: 2C₂H₆(g) + 7O₂(g) → 6H₂O(g) + 4CO₂(g). Calculate the volume of steam that would be produced.
C₂H₆ : H₂O = 2 : 6. V = (400 × 6)/2 = 1200 cm³.
27. A saturated solution at 30 °C will normally produce crystals at a temperature of
A saturated solution at 30°C produces crystals when cooled to any temperature below 30°C.
28. The following organic compounds are polymers except
Rubber, starch and proteins are polymers; fats are triglycerides (three fatty-acid chains on glycerol), not polymers of repeating units.
29. According to the collision theory of reaction rates, which of the following conditions is not required for two molecules to react?
Collision theory requires sufficient energy and correct orientation, but does not require the collision itself to be perfectly elastic (no energy loss).
30. A weak acid is one which
A strong acid ionizes completely in solution; a weak acid ionizes only partially (slightly).
31. Which of the following conditions are necessary for the preparation of alkanoates from alkanols and alkanoic acids?
Esterification (making alkanoates) requires an acid catalyst (typically conc. H₂SO₄) and heat.
32. The volume of 22 g of CO₂ at stp is equivalent to [C=12, O=16, molar volume at stp = 22.4 dm³]
Mole of CO₂ = 22/44 = 0.5 mol. Volume at stp = 0.5 × 22.4 = 11.2 dm³.
33. Pairs of outermost shell electrons which are not used in bonding are
Lone pairs are valence electron pairs not involved in bonding; they influence molecular shape and reactivity.
34. ₈X²⁻ and ₁₀Y are
₈X²⁻ has 8+2=10 electrons; ₁₀Y has 10 electrons. Species with equal electron counts are isoelectronic.
35. Pure water contaminated with quicklime will have a pH of
Quicklime (calcium oxide) is basic, so contaminated water will have a pH greater than 7.
36. The electron configuration of ₂₉Cu is
Copper adopts 4s¹3d¹⁰ rather than 4s²3d⁹ because the fully filled 3d subshell offers greater stability.
37. When s and p block elements react, the bond formed is
s-block metals lose electrons to become cations while p-block non-metals gain electrons to become anions, forming an electrovalent (ionic) bond.
38. The hardest form of carbon is
Diamond's tetrahedral, fully cross-linked lattice structure makes it the hardest natural form of carbon.
39. The metallic bonding in aluminium is strong because of
Each aluminium atom contributes three valence electrons to a dense sea of delocalized electrons, giving strong metallic bonding.
40. How many covalent bonds are formed by Nitrogen?
Nitrogen has 5 valence electrons and needs 3 more to complete its octet, so it forms 3 covalent bonds.
41. When an element exists in two or more forms in the same physical state, it exhibits
Allotropy describes an element existing in multiple structural forms within the same physical state, e.g. carbon as diamond, graphite, graphene.
42. An atom of an element in the ground state contains 8 valence electrons. The element is considered as a
A complete octet of valence electrons (8, except He with 2) is characteristic of the stable, chemically inert noble gases.
43. The main function of limestone in the blast furnace during the extraction of iron is to
Limestone decomposes to CaO, which reacts with silica impurities to form slag (CaSiO₃), removing them from the molten iron.
44. Which of the following is a monomer of polythene?
Ethene molecules undergo polymerization to form the long chains of polythene.
45. Which of the following compounds is formed by the oxidation of ethanol?
Complete oxidation of ethanol produces ethanoic acid, CH₃COOH.
46. Tetraoxosulphate (VI) acid is considered as a heavy chemical because
H₂SO₄ is termed a 'heavy chemical' because it is manufactured in very large quantities and underpins many industrial processes.
47. How many isomers can be formed from a compound with molecular formula C₅H₁₂?
C₅H₁₂ (pentane) has three isomers: n-pentane, 2-methylbutane, and 2,2-dimethylpropane.
48. The oxidation number of iron in its free state is
The oxidation number of any element in its free (uncombined) state is zero.
49. Water pipes are produced from
PVC is durable, light, inexpensive and corrosion-resistant, making it ideal for water pipes.
50. Consider the reaction xAl + yCl₂ → zAlCl₃. The values of x, y and z respectively are
The balanced equation is 2Al + 3Cl₂ → 2AlCl₃, so x=2, y=3, z=2.
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