All 50 questions from the West African Examinations Council (WAEC) Chemistry 2018 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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Which of the following raw materials is used in the plastic industry?
Ethene is polymerised to form polythene, making it a key raw material for the plastic industry.
Which of the following organic compounds can undergo both addition and substitution reactions?
Benzene, despite its ring stability, can undergo both substitution (its typical reaction) and addition reactions under certain conditions; the alkanes (pentane, propane, hexane) are saturated and can only undergo substitution.
Which of the following equations represents a redox reaction?
In 2H₂S+SO₂→2H₂O+3S, sulphur's oxidation state changes in both reactants (H₂S is oxidised, SO₂ is reduced), making it the redox reaction; the others are simple precipitation, neutralisation, or decomposition.
The process of extraction of iron from its ore is by
Iron is extracted from its ore (haematite) by chemical reduction using carbon monoxide in the blast furnace.
What is the solubility of a salt if 0.4g of it is obtained on evaporating 200cm³ of its saturated solution to dryness?
Solubility = mass/volume(dm³) = 0.4g / 0.2dm³ = 2.0 gdm⁻³.
An acidic salt has a double anions in its aqueous solution
An acidic salt (e.g. NaHSO₄, NaHS) releases hydrogen ions (H⁺) in aqueous solution, in addition to its normal cation and anion.
A reaction is endothermic if the
In an endothermic reaction, heat is absorbed from the surroundings, so the reaction vessel feels cold to the touch.
In which of the following compounds does hydrogen form ionic compound?
In NaH, hydrogen exists as the H⁻ ion, forming an ionic (electrovalent) bond with sodium; in CH₄, HCl and NH₃ hydrogen is covalently bonded.
Consider the following reaction equation: Br₂ + 2KI → 2KBr + I₂. Bromine is reacting as a(n)
The oxidation number of bromine decreases from 0 (in Br₂) to −1 (in KBr) — bromine is reduced, meaning it acts as the oxidizing agent.
An organic compound has the empirical formula CH. If its molar mass is 42 gmol⁻¹, what is its molecular formula? [H=1.0, C=12.0]
Empirical formula mass CH = 13. n = 42/14... using (CH)n=42 → n=42/13≈3, so molecular formula = C₃H₆ (mass 42).
Ethene is produced from ethanol by
Ethanol undergoes dehydration (loss of water) using excess concentrated H₂SO₄ at about 180°C to produce ethene.
Consider the following equilibrium reaction: 2AB(g) + B₂(g) ⇌ 2AB₂(g) ΔH = −X kJmol⁻¹ the backward reaction will be favoured by
Since the backward reaction proceeds with an increase in volume (moles of gas), it is favoured by a decrease in pressure.
What is the mass of solute in 500cm³ moldm⁻³ H₂SO₄? [H=1.0, O=16.0, S=32.0]
Volume = 500/1000 = 0.5dm³; concentration 0.005moldm⁻³ gives 0.0025mol; mass = 0.0025 × 98 = 0.245g.
Pure water can be made to boil at a temperature lower than 100°C by
The boiling point of water decreases with decreasing external (atmospheric) pressure.
Consider the following sketch of the solubility curve of some substances. At what temperature does the solubility of KNO₃ equal that of NaNO₃?
The solubility of KNO₃ equals that of NaNO₃ at the temperature where their solubility curves intersect — about 20°C.
When a salt is added to its saturated solution, the salt
In a saturated solution, no more solute can dissolve, so any additional salt added simply precipitates out, leaving the solution unchanged.
When substance X was added to a solution of bromine water, the solution became colourless. X is likely to be
A compound that decolourises bromine water must be unsaturated (an alkene or alkyne); propyne (an alkyne) fits this.
The preferential discharge of ions during electrolysis is influenced by the
Preferential discharge is influenced by factors including the position of the ion in the electrochemical series, concentration, and the nature of the electrode.
The valence electrons of ₁₂Mg are in the
Magnesium (Z=12): 1s²2s²2p⁶6 3s². Its valence electrons occupy the 3s orbital.
Stainless steel is an alloy comprising of
Stainless steel is primarily an alloy of iron and carbon (with chromium and nickel typically added); of the given options, Fe and C best represents its base composition.
The number of hydrogen ions in 1.0dm³ of 0.02moldm⁻³ tetraoxosulphate(VI) acid is [N_A=6.02×10²³]
moles H₂SO₄ = 0.02mol → moles H⁺ = 0.04mol. Number of ions = 0.04 × 6.02×10²³ = 2.4×10²².
The most suitable substance for putting out petrol fire is
Carbon (IV) oxide (CO₂) is the most suitable extinguishing agent for petrol fires as it smothers the flame without spreading burning fuel (unlike water).
The following factors would contribute to environmental pollution except
Photosynthesis actually consumes CO₂ (a greenhouse gas) and reduces pollution, unlike the other listed industrial/combustion processes.
The position of equilibrium in a reversible reaction is affected by
The position of equilibrium is affected by concentration, pressure, and temperature changes — not by particle size, stirring, or catalysts (which only affect rate).
The diagram below illustrates a conical flask containing water and ice. Which of the following statements about the diagram is correct?
Melting is an endothermic process — energy is absorbed from the surroundings as the ice changes to water.
Which of the following statements best explains the differences between a gas and a vapour?
A vapour, unlike a true gas, can be liquefied by pressure alone at room temperature, since it exists below its critical temperature.
Consider the following reaction equations: 2HCl+Ca(OH)₂→CaCl₂+2H₂O. What is the volume of 0.1 moldm⁻³ HCl that would completely neutralize 25cm³ of 0.3 moldm⁻³ Ca(OH)₂?
Using CaVa/na = CbVb/nb with na=2 (HCl), nb=1 (Ca(OH)₂): Va = (0.3×25×2)/(0.1×1) = 150cm³.
Cu and HNO₃ are not suitable for preparing hydrogen gas because of their
Copper is too unreactive (below hydrogen in the electrochemical series) to displace hydrogen, while HNO₃'s strong oxidizing nature converts any hydrogen produced into water instead.
Which of the following formula cannot be an empirical formula?
An empirical formula must be in its simplest whole-number ratio; N₂O₄ can be reduced further to NO₂, so it is not an empirical formula.
One of the criteria for confirming the purity of benzene is to determine its
The purity of a liquid such as benzene can be confirmed by its boiling point — presence of impurity raises the boiling point above the pure substance's known value.
When chlorine is passed through a sample of water, the pH of the water sample would
Chlorine is an acidic gas; when passed into water it forms HCl and HOCl, lowering the pH below 7.
How many atoms are contained in 0.2 moles of nitrogen? [N_A=6.02×10²³]
1 mole of N₂ contains 2 moles of N atoms. 0.2mol N₂ → 0.4mol N atoms. Number = 0.4×6.02×10²³ = 2.41×10²³.
The strength of metallic bonds depends on the
The strength of a metallic bond varies directly with the number of valence electrons contributed to the delocalised 'sea of electrons', other factors being constant.
When Zinc is added to AgNO₃ solution, crystals of silver form on surface. This indicates that zinc is
Zinc loses electrons to displace silver from solution (Zn → Zn²⁺ + 2e⁻), meaning zinc is oxidized.
The empirical formula of a compound containing 0.067 mol Cu and 0.066 mol O is [Cu=63.5, O=16.0]
Mole ratio Cu:O ≈ 0.067:0.066 ≈ 1:1, giving empirical formula CuO.
The change in the oxidation state of iron in the reaction represented by the equation below is 2FeCl₃+H₂S→2FeCl₂+2HCl+S
Iron changes from the +3 oxidation state in FeCl₃ to the +2 state in FeCl₂.
Which of the following methods can be used to separate blood cells from plasma?
Blood cells can be separated from plasma by centrifugation, which spins the denser cells to the bottom.
Which of the following statements about ionic radius is correct? Ionic radius
Across a period, as nuclear charge increases, the increased pull on electrons causes ionic (and atomic) radius to decrease.
Analysis of a hydrocarbon shows that it contains 0.93g of carbon per gram of the compound. The mole ratio of carbon to hydrogen in the compound is [H=1.0, C=12.0]
Carbon = 0.93g, Hydrogen = 1−0.93 = 0.07g. Moles: C=0.93/12≈0.0775, H=0.07/1=0.07. Ratio ≈ 1:1.
The law of definite proportions states that
The law of definite proportions (constant composition) states that a pure compound always contains the same elements combined in the same fixed proportion by mass.
What is the relative molecular mass of a compound which has empirical formula CH₂O? [H=1.0, C=12.0, O=16.0]
Empirical formula mass CH₂O = 30. Testing the given options, only 180 is a whole-number multiple of 30 (180=30×6), so the molecular mass is 180.
Atoms are electrically neutral because they
An atom is neutral because the number of positively charged protons equals the number of negatively charged electrons.
Common salt (NaCl) is used for preserving foods. Which of the following properties could be used to determine its purity before use?
The purity of a solid such as NaCl can be assessed by its melting point — a pure sample melts sharply at a known temperature, while an impure one melts over a range.
Which of the following electron configurations represents the transition element chromium (₂₄Cr)?
Chromium is an exception to the normal filling order: its actual configuration is [Ar]4s¹3d⁵ (one electron shifts from 4s to 3d for a more stable half-filled d-subshell).
The atomic number of an isotope of hydrogen is equal to its mass number. This isotope is
Protium (¹₁H), the most common isotope of hydrogen, has 1 proton and 0 neutrons, so its atomic number equals its mass number (both = 1).
The total number of shared pairs of electrons in the compound shown below (H₂C=CH₂, ethene) is
Ethene (H₂C=CH₂) has 4 C–H single bonds (4 shared pairs) plus 1 C=C double bond (2 shared pairs) = 6 shared pairs of electrons.
The bonding pair of electrons in a hydrogen chloride molecule is pulled towards the chlorine atom because
Chlorine is more electronegative than hydrogen, so it pulls the shared bonding electron pair closer to itself.
The solubility of CO₂ in water can be accounted for by
CO₂ dissolves in water partly due to dipole (dipole-induced dipole) attraction between the polar water molecules and the CO₂ molecule.
Which of the following properties would not influence electrovalent bond formation?
Electrovalent (ionic) bond formation depends on electronegativity, electron affinity and ionization energy — catalytic ability has no bearing on bond formation.
Particles in a solid exhibit
In a solid, particles are fixed in position relative to each other and can only vibrate about these fixed points — they cannot move translationally.
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