All 50 questions from the West African Examinations Council (WAEC) Chemistry 2023 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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Which of the following laws or theory cannot be explained by the application of the kinetic theory of gases?
As they are gas laws, Charles' law, Gay-Lussac's law and Boyle's law can be explained by the kinetic theory of gases. Dalton's atomic theory is not a gas law.
The primary component of natural gas is
Methane is the main component of natural gas. It makes up 70–90% of natural gas. The other significant components are ethane, propane and butane; nitrogen, carbon dioxide, helium and trace amounts of hydrogen sulphide and odorants can also be present.
Thermal cracking of alkanes usually
In thermal cracking, high temperatures (usually 450–750°C) and pressures (up to about 70 atmospheres) are used to break large hydrocarbons into smaller ones — this is a decomposition process.
Carbon is deposited in the exhaust pipes of vehicles because of
The presence of carbon deposits in the exhaust pipes of cars is a result of incomplete combustion of the fuel.
How many moles of copper would be deposited by passing 1 Faraday of electricity through a CuCl2 solution?
Cu2+ + 2e− → Cu. Since 2 moles of electrons (2F) are required to deposit 1 mole of copper, the amount deposited using 1F = (1 mol/2F) × 1F = 0.5 mol.
Alkenes can be manufactured by
Alkenes can be obtained from the cracking of hydrocarbons. (Combustion of alkanes produces CO2 and H2O; addition of hydrogen to vegetable oils produces margarine, using a nickel catalyst at about 200°C.)
Petrochemistry is an example of
Petrochemistry is a branch of applied chemistry that focuses on how crude oil and natural gas are transformed into raw materials and other useful products.
Species that occur in a reaction pathway but not in the overall reaction are known as
Species that occur in a reaction pathway but not in the overall (net) reaction equation are known as intermediates.
Which of the following statements is correct?
Since the general formula of alkanes is CnH2n+2, an alkane with 49 carbon atoms will contain (2×49)+2 = 100 hydrogen atoms. The reaction between alkanes and chlorine is a substitution reaction, not addition. Butane is heavier than propane so has a higher boiling point than propane (not relevant to this comparison — butane's boiling point is in fact higher than propane's, and lower than pentane's, so option C is false). Pentane has three isomers, not five.
The best indicator to use for the titration of ethanoic acid with sodium hydroxide is
A titration involving a weak acid (such as ethanoic acid) and a strong base (such as sodium hydroxide) requires phenolphthalein as the indicator.
The reduction half equation of the following reaction is: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Zn + CuSO4 → ZnSO4 + Cu. Ionically: Zn + Cu2+ → Zn2+ + Cu. Reduction half equation: Cu2+ + 2e⁻ → Cu. (Oxidation half equation: Zn → Zn2+ + 2e⁻.)
If 100 cm3 of a saturated solution of sodium tetraoxosulphate (VI) at 30°C contains 10.5 g of the salt, what would be its solubility at this temperature? [Na2SO4 = 142]
Mole = 10.5 g ÷ 142 g/mol = 0.0739 mol. Since 0.0739 mol saturates 100 cm3 at 30°C, 1000 cm3 (1 dm3) at the same temperature contains (0.0739/100)×1000 = 0.739 mol. Solubility ≈ 0.74 mol dm⁻³.
An example of a crystalline substance that does not possess water of crystallization is
Examples of salts without water of crystallization include potassium nitrate, barium sulphate and sodium nitrate.
The salt solution formed from a reaction between ethanoic acid and sodium hydroxide solution would be
A salt solution formed from a reaction between a weak acid (such as ethanoic acid) and a strong base (such as sodium hydroxide) will be alkaline/basic. (If the reaction were between a strong acid and a weak base, the resulting salt solution would be acidic.)
Which of the following reactions represents the hydrolysis of an alkanoate?
The hydrolysis of an alkanoate (ester) yields the constituent alkanoic acid and alkanol: RCOOR' + H2O ⇌ RCOOH + R'OH. For example, hydrolysis of ethyl ethanoate: CH3COOCH2CH3 + H2O ⇌ CH3COOH + CH3CH2OH.
Which of the following statements about the collision theory is correct?
Only effective collisions bring about reactions. The higher the number of such collisions, the higher the reaction rate.
Why are H2SO4 and CaCl2 not suitable for drying ammonia gas? They
H2SO4 and CaCl2 are not suitable for drying ammonia because they react with the gas. As a rule, an alkaline gas should not be dried with an acidic substance, and an acidic gas should not be dried with an alkaline substance.
Which of the following equations does not illustrate correctly one of the reactions of chlorine?
Fluorine is more reactive than chlorine, so chlorine cannot displace fluorine from sodium fluoride, as portrayed in option C.
How many unpaired electrons are present in ₂₆Fe³⁺?
₂₆Fe = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶. The two 4s electrons and one 3d electron are removed to form Fe³⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵. The five 3d electrons are all unpaired, so Fe³⁺ has five unpaired electrons.
Going down group II in the periodic table normally
Going down group II: shielding effect increases; melting point decreases; ionization energy decreases; electronegativity decreases. Only 'shielding effect increases' (option A) is a correct trend among those listed — the other three options describe the opposite of the actual trend.
Which of the following elements has its valence electrons in the s-orbital?
₁₁Na = 1s² 2s² 2p⁶ 3s¹; ₆C = 1s² 2s² 2p²; ₁₅P = 1s² 2s² 2p⁶ 3s² 3p³; ₁₃Al = 1s² 2s² 2p⁶ 3s² 3p¹. Sodium has its valence electron in the s-orbital, while the others have valence electrons in both s and p orbitals.
The periodic property that is used to determine whether a covalent molecule is polar or not is
Electronegativity is used to investigate the nature of a bond: if the difference between the electronegativities of the bonded atoms is less than 0.5, the bond is non-polar covalent; from 0.5 to 1.7, it is polar covalent; greater than 1.7, it is ionic.
The following steps are scientific methods except
Open-mindedness is a scientific attribute, not a scientific method in itself.
Isoelectronic species have the same number of
Isoelectronic species have the same number of electrons. For example, Ne, Na⁺, Mg²⁺, Al³⁺, F⁻, O²⁻ and N³⁻ each contain 10 electrons and are therefore isoelectronic.
An element X has two isotopes, ⁶⁵₃₀X and ⁶⁶₃₀X with relative abundance of 60% and 40% respectively. The relative atomic mass of X is
R.A.M. = (mass₁×%₁ + mass₂×%₂)/100 = (65×60 + 66×40)/100 = (3900+2640)/100 = 6540/100 = 65.4
The pair of compounds that belongs to the same homologous series is
C3H8 (propane, an alkane) and C3H6 (propene, an alkene) do not belong to the same series; same for C4H10 (butane, alkane) and C5H10 (pentene, alkene); and for C2H4 (ethene, alkene) and C4H10 (butane, alkane). C2H6 (ethane) and C4H10 (butane) both belong to the alkane family, so they are in the same homologous series.
A consequence of global warming is
Flooding is a major consequence of global warming.
Which of the following gases has the lowest rate of diffusion? [H=1.0, C=12.0, N=14.0, O=16.0]
N2 = 14×2 = 28; NH3 = 14+(1×3) = 17; O2 = 16×2 = 32; CH4 = 12+(1×4) = 16. The heavier a gas, the lower its rate of diffusion. Oxygen is the heaviest here, so it has the lowest diffusion rate.
The gas that is less dense than air is
For a gas to be less dense than air, its RMM should be less than 28.8 (approx. average molar mass of air). CO2 = 44, N2 = 28, Cl2 = 71, O2 = 32. Only nitrogen is less dense than air here.
Which of the following equimolar solutions has the highest conductivity?
H2SO4 is a strong acid that fully dissociates, giving it the highest conductivity. H2CO3 has the lowest conductivity since it is a weak acid that only dissociates slightly.
What takes place at the cathode during electrolysis?
At the cathode, reduction occurs — cations gain electron(s) and are discharged. (At the anode, oxidation occurs — anions lose electron(s) and are discharged.)
How many grammes of NaOH(s) would be needed to produce 100.0 cm3 of a 0.20 mol dm⁻³ NaOH(aq)? [NaOH=40.0]
Volume = 100 cm3 = 0.1 dm3. Mole of NaOH = 0.20 mol dm⁻³ × 0.1 dm3 = 0.02 mol. Mass = 0.02 mol × 40 g/mol = 0.80 g.
The formation of a bond between hydrogen and a highly electronegative atom results in
Between hydrogen and a very small, highly electronegative atom (nitrogen, oxygen or fluorine), a hydrogen bond is formed — hydrogen bonding results from bond polarity.
The molecule that has a non-polar covalent bond is
Since the electronegativity difference between the two identical chlorine atoms in Cl2 is zero, the bond is non-polar. The same applies to F2, Br2, I2, N2, O2 and H2.
Use the following information to answer this question. Which of the elements in the table below would react more readily with chlorine?
Chlorine (being electronegative) reacts readily with elements that lose electrons easily. A higher ionization energy means it is harder to lose an electron. Since W and X have the lowest ionization energies in the table, they lose electrons most easily and so react most readily with chlorine.
The relative molar mass of a gaseous hydrocarbon is 30. Determine its vapour density.
Vapour density = ½ × Relative molecular mass = ½ × 30 = 15.
Consider the following reaction equation: 2SO3(g) → 2SO2(g) + O2(g); ΔH = +198 kJ mol⁻¹. Which of the following statements about the reaction is correct?
Since ΔH is positive, the reaction is endothermic, not exothermic — heat is absorbed, not released. An endothermic reaction container would feel colder, not warmer.
Which of the following properties does not give evidence of the kinetic theory of matter?
Evaporation, diffusion and melting give evidence of the kinetic theory of matter (particles in constant motion); polymerization does not.
A compound that could be dried by using conc. tetraoxosulphate (VI) acid and not by calcium oxide is likely to be
It is likely to be an acid anhydride (an acidic gas). An acidic gas should not be dried with an alkaline/basic drying agent (like calcium oxide) since they would react — but it can be dried with an acidic drying agent such as concentrated H2SO4. (See the reasoning for Q17.)
Positive ions in a sea of electrons are found in
In a metallic bond, there is collective sharing of a 'sea' of delocalised valence electrons among positively charged metal ions.
Dilute trioxonitrate (V) acid does not produce hydrogen when it reacts with metals because
HNO3 does not liberate hydrogen gas when it reacts with metals (unlike other acids) because it is a strong oxidizing agent — any hydrogen formed is immediately oxidized to water.
How many moles are there in 3.0 g of O3? [O=16.0]
Molar mass of O3 = 16×3 = 48 g/mol. Mole = 3 g ÷ 48 g/mol = 0.0625 mol.
Copper (II) ions are able to participate in co-ordinate covalent bonding because they
Copper (II) ions can participate in co-ordinate (dative) covalent bonding because they have vacant d-orbitals that can accept electron pairs.
Determine the volume of 0.100 mol of HCl in 0.250 mol dm⁻³ solution.
Mole = Concentration (mol dm⁻³) × Volume (dm3). 0.1 mol = 0.25 mol dm⁻³ × V, so V = 0.1/0.25 = 0.4 dm3 = 400 cm3.
Which of the statements about gases is not correct?
Unlike most solids, the solubility of most gases in water decreases as temperature increases (and vice versa) — so option D is the incorrect statement.
Arrange the following compounds in decreasing order of their boiling points: NH3, HF, SiH4, CH4
NH3 and HF have higher boiling points than SiH4 and CH4 because NH3 and HF exhibit hydrogen bonding. HF has the strongest hydrogen bonding of common substances, so its boiling point is higher than NH3's. SiH4 has a higher boiling point than CH4 because it is heavier. Decreasing order: HF > NH3 > SiH4 > CH4.
The following statements are correct except
Solids are substantially incompressible, so a large decrease in volume is not expected when pressure is applied to a solid metal — option D is incorrect.
The vapour density of an organic compound with the molecular formula C2H4O2 is [H=1.0, C=12.0, O=16.0]
C2H4O2 = (12×2)+(1×4)+(16×2) = 60. Vapour density = ½ × RMM = ½ × 60 = 30.
A mixture containing two salts of different solubility can be separated by
A mixture of two salts with different solubilities can be separated by fractional crystallization.
The separation technique that is suitable for separating iodine from tetrachloromethane is
Solvent extraction is a separation technique that relies on the differential solubility of a substance in two immiscible solvents — suitable for separating iodine dissolved in tetrachloromethane.
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