WAEC Chemistry 2023
Questions & Answers

A student might mistakenly choose option B, C, or D thinking that kinetic theory applies to fewer gas properties than it actually does. The kinetic theory of gases explains gas behavior through molecular motion, successfully accounting for pressure-volume relations in Boyle's law, volume-temperature relations in Charles' law, and pressure-temperature relations in Gay-Lussac's law. Dalton's atomic theory, however, addresses the composition of matter into indivisible atoms, a concept not directly derived from gas kinetics. Common mistake: grouping atomic composition theories together with gas laws.

One might mistakenly select option A, B, or D due to familiarity with various short-chain hydrocarbons present in petroleum fractions. Natural gas primarily consists of methane, making up about 70 to 90 percent of its composition, alongside smaller quantities of ethane, propane, and butane. Because methane acts as the main fuel source, it establishes natural gas as a clean-burning fossil fuel. Common mistake: assuming the minor hydrocarbon constituents represent the primary bulk of natural gas.

A student might mistakenly select option B or C believing that cracking releases heat or yields exclusively light alkanes. Thermal cracking breaks down large alkane molecules into smaller hydrocarbons through heat-induced decomposition, creating a mixture of alkenes and smaller alkanes utilized for fuels like petrol. This process is endothermic and does not require hydrogen, distinguishing it from catalytic methods. Common mistake: confusing thermal cracking's endothermic nature with exothermic combustion reactions.

A test-taker might mistakenly select option B, assuming that because carbon is a constituent of fuel molecules, it naturally deposits directly in the exhaust. Carbon is deposited in the exhaust due to the incomplete combustion of petrol under low-oxygen conditions, which produces carbon monoxide and soot instead of clean carbon dioxide and water vapor. Common mistake: confusing normal fuel composition with incomplete combustion byproducts.

A student might mistakenly pick option B or A by failing to account for the ionic charge or the stoichiometry of the reduction half-reaction. One faraday represents 1 mole of electrons, and the reduction at the cathode proceeds via Cu²⁺ + 2e⁻ → Cu, meaning 2 faradays of electricity are required to deposit 1 mole of copper. Consequently, passing 1 faraday deposits half that amount, yielding 0.5 moles. Common mistake: forgetting to balance the electron stoichiometry for divalent metal ions during electrolysis.

One might mistakenly select option A or C by confusing alkene production pathways with fat saturation or general polymerization. Alkenes are manufactured via the cracking of hydrocarbons, which breaks carbon-carbon bonds in larger alkanes to yield smaller alkanes and alkenes, such as converting butane into ethene and ethane. Hydrogenation turns oils into fats, polymerization links monomers, and combustion oxidizes compounds into carbon dioxide and water. Common mistake: confusing the thermal breaking of alkanes with the addition reactions used to saturate vegetable oils.

A student might mistakenly choose option A or C assuming that any large-scale chemical processing falls under pure research or biochemistry. Petrochemistry is an example of applied chemistry because it utilizes chemical principles to transform petroleum into practical industrial products like plastics and fuels rather than investigating fundamental theories. Common mistake: confusing industrial application fields with pure or biological branches of science.

A student might mistakenly select option A or C by confusing transient species with final products or initial starting materials. Intermediates are transient chemical species that form during a reaction pathway and are subsequently consumed, meaning they do not appear in the overall net equation, such as carbocations in SN1 reactions. Products represent the final outcome, reactants are initial materials, and inhibitors function to slow down reactions. Common mistake: listing reaction intermediates as final products in a net chemical equation.

A student might mistakenly pick option B, C, or D due to misconceptions about homologous series formulas or boiling point trends. Alkanes follow the general formula CₙH₂ₙ₊₂, meaning an alkane with 49 carbon atoms correctly contains 100 hydrogen atoms. Alkanes undergo substitution rather than addition with chlorine, higher molecular weight alkanes like butane have higher boiling points than propane due to increased electrons, and pentane has 3 isomers rather than five. Common mistake: misapplying general molecular formulas for aliphatic hydrocarbons.

A student might mistakenly choose option A or B because methyl red and methyl orange are common acid-base indicators. For the titration of ethanoic acid, which is a weak acid, with sodium hydroxide, a strong base, the equivalence point occurs at a basic pH around 8.7. Phenolphthalein is the best indicator because its color change range of 8.2 to 10 closely matches this endpoint, whereas methyl red and methyl orange change too early in the acidic range. Common mistake: using strong-acid indicators for weak-acid strong-base titrations.

A student might mistakenly select option D by choosing the oxidation half-reaction instead of the reduction process. The reduction half-equation represents the gain of electrons at the cathode, where copper ions gain two electrons to form solid copper according to Cu²⁺(aq) + 2e⁻ → Cu(s). Option A describes salt dissociation, and option C uses incorrect electron stoichiometry. Common mistake: confusing the oxidation half-reaction of the zinc electrode with the reduction of copper ions.

A student might mistakenly select option A, B, or D through calculation errors involving molar mass or volume conversions. To find the solubility, first calculate the moles of solute by dividing 10.5 g by the molar mass of 142 g/mol, which equals 0.074 moles. Then, divide this mole value by the volume of 0.1 dm³ (converted from 100 cm³) to yield a solubility of 0.74 mol dm⁻³. Common mistake: omitting the volume conversion from cubic centimeters to cubic decimeters.

A student might mistakenly pick option B, C, or D because hydrated crystal forms are common among many common salts. Potassium trioxonitrate(V), or KNO₃, is an anhydrous crystalline substance that does not possess water of crystallization in its chemical formula. In contrast, sodium carbonate, iron(II) sulfate, and sodium sulfate often exist in hydrated forms containing water molecules. Common mistake: assuming all crystalline salts contain water of crystallization.

One might mistakenly select option B or C, assuming that all reactions involving acids automatically yield acidic solutions or that neutralization creates neutral conditions. The salt solution formed from ethanoic acid, a weak acid, and sodium hydroxide, a strong base, is basic because the resulting salt undergoes hydrolysis where the acetate ion reacts with water to produce hydroxide ions. Common mistake: assuming neutral pH for salts derived from weak acids and strong bases.

A student might mistakenly select option A, which represents esterification rather than hydrolysis. The correct representation for the hydrolysis of an alkanoate is CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻, where the acetate ion reacts with water to yield ethanoic acid and hydroxide ions, thereby creating a basic solution. Common mistake: confusing the reverse esterification reaction with salt hydrolysis.

A student might mistakenly select option A or D by assuming that every molecular impact leads to a chemical change. Collision theory states that the rate of a reaction is directly proportional to the number of effective collisions, which require both sufficient activation energy and correct spatial orientation. Ineffective collisions simply fail to bring about a chemical reaction. Common mistake: assuming all physical collisions between reactant molecules result in product formation.

A student might mistakenly choose option A, assuming that strong drying agents are avoided simply because they are corrosive. Sulfuric acid and calcium chloride are unsuitable for drying ammonia gas specifically because they react chemically with the gas, forming stable addition compounds such as ammonium sulfate and calcium chloride adducts. Common mistake: attributing the unsuitability of a desiccant to its corrosive nature rather than its chemical reactivity with the target gas.

Distractor check: A student might mistakenly select option B or D due to their familiarity as standard industrial preparations and bleaching reactions, or choose A since chlorine acts as an oxidizing agent there. Reasoning to the answer: Fluorine is more electronegative and reactive than chlorine, meaning chlorine cannot force fluorine out from sodium fluoride. Therefore, option C fails to describe a valid chemical displacement process, whereas the other choices correctly demonstrate chlorine bleaching hydrogen sulfide gas and creating sodium hypochlorite or bleaching powder. Common mistake: Assuming all halogens can displace one another indiscriminately regardless of their position in the electrochemical series.

Distractor check: A student might pick options A, B, C, or D by incorrectly configuring neutral iron or miscalculating the electrons lost during oxidation. Reasoning to the answer: A neutral iron atom possesses the electronic configuration 3d⁶4s². Removing three electrons to form the Fe³⁺ ion strips both 4s electrons and one 3d electron, leaving a 3d⁵ configuration. Distributing these remaining five electrons across five distinct orbitals according to Hund's rule results in five unpaired electrons, aligning with option E. Common mistake: Forgetting to remove electrons from the outermost 4s shell before taking them from the 3d sublevel when ionizing transition metals.

Distractor check: A student might mistakenly pick option A, C, or D assuming that atomic properties universally decrease down a group just like atomic size increases. Reasoning to the answer: Moving downward through group II, the atomic radii expand significantly, which brings an increase in the number of delocalized electrons participating in metallic binding. This enhanced metallic bonding causes melting points to rise overall, even though there are minor irregularities across the group. Common mistake: Confusing the trends of atomic shielding, ionization energy, and electronegativity—which decrease down a group—with the behavior of metallic melting points.

Distractor check: A student might choose options B, C, or D by confusing the valence shell configuration of alkali metals with those of other representative elements. Reasoning to the answer: Sodium has the atomic number 11 with an electronic configuration of [Ne]3s¹, placing its outermost valence electrons explicitly within the s-orbital. In contrast, carbon fills the p-orbital with [He]2s²2p², phosphorus terminates in [Ne]3s²3p³, and aluminum concludes with [Ne]3s²3p¹. Common mistake: Overlooking the exact subshell of the outermost principal energy level and guessing based on period position.

Distractor check: A student might mistakenly select atomic radius or ionization energy, confusing general periodic trends with bond polarity determinants. Reasoning to the answer: Evaluating whether a covalent molecule is polar relies directly on electronegativity values, where an electronegativity difference between 0.4 and 1.7 creates an uneven charge distribution, exemplified by the polar molecule HCl. Electronegativity dictates how strongly bonded atoms attract shared electron pairs, directly producing molecular dipoles. Common mistake: Assuming atomic radius or electron affinity directly accounts for charge separation within a covalent bond.

Distractor check: A student might mistakenly choose analysis, experiment, or problem identification, thinking those are non-scientific elements. Reasoning to the answer: The formal scientific method strictly follows sequential progression steps including problem identification, forming hypotheses, conducting experiments, and analyzing gathered data. Open-mindedness is instead an intrinsic personal attitude or virtue of a researcher, rather than a procedural step in the methodology. Common mistake: Confusing scientific character traits and attitudes with procedural actions in an experiment.

Distractor check: A student might mistakenly choose options B, C, or D by confusing isoelectronic particles with isotopes or nuclides that share protons and neutrons. Reasoning to the answer: Isoelectronic chemical species share identical electronic configurations and total electron counts, such as Na⁺, Ne, and F⁻ which all contain 10 electrons, while differing entirely in their nuclear proton and neutron counts. Common mistake: Mistaking isoelectronic structures for atomic isotopes that share identical numbers of protons.

Distractor check: A student might pick options A, B, or C by incorrectly averaging the isotope masses or miscalculating the weighted fractions. Reasoning to the answer: The relative atomic mass is calculated by multiplying each isotope's mass by its fractional abundance and summing them up, yielding (65 × 0.3) + (67 × 0.7), which equals 19.5 + 46.9 = 66.4, matching option D when rounded to the appropriate context. Common mistake: Simply taking the arithmetic mean of the mass numbers instead of calculating a percentage-weighted average.

Distractor check: A student might select options A, B, or C by failing to check the general molecular formula of hydrocarbons or mixing alkanes and alkenes. Reasoning to the answer: Both ethane (C₂H₆) and butane (C₄H₁₀) belong to the alkane family, obeying the general molecular formula CₙH₂ₙ₊₂. The other options mix alkanes with alkenes or pair mismatched carbon-hydrogen ratios. Common mistake: Assuming any two hydrocarbons containing carbon and hydrogen automatically belong to the same organic series.

Distractor check: A student might select options A, B, or C by misunderstanding the acid-base interactions between drying agents and specific gas types. Reasoning to the answer: Acid anhydrides such as sulfur trioxide react with basic calcium oxide to form salts and therefore cannot be dried with it, but they remain inert toward concentrated sulfuric acid, allowing safe moisture removal. Alkali gases react with sulfuric acid, and deliquescent salts absorb water indiscriminately from both. Common mistake: Assuming all chemical drying agents are universally interchangeable regardless of the chemical nature of the gas.

Distractor check: A student might mistakenly pick nitrogen, ammonia, or methane by guessing based on common gas names rather than calculating molar masses. Reasoning to the answer: According to Graham's law, the rate of diffusion is inversely proportional to the square root of the relative molar mass. Calculating the molar masses gives ammonia at 17, methane at 16, nitrogen at 28, and oxygen at 32, meaning oxygen possesses the highest molar mass and thus the slowest diffusion rate. Common mistake: Confusing inverse proportionality with direct proportionality when relating molecular weight to diffusion speed.

Distractor check: A student might choose options A, C, or D, thinking all common laboratory gases are heavier than air. Reasoning to the answer: Nitrogen gas has a density of approximately 1.25 g/L, which is slightly lighter than standard air density of about 1.29 g/L. Conversely, carbon(IV) oxide, chlorine, and oxygen exhibit significantly higher densities than air. Common mistake: Forgetting standard air density values and assuming atmospheric gases share identical physical weights.

Distractor check: A student might mistakenly pick NaOH(aq), forgetting that strong diprotic acids dissociate into more ions than strong monobasic alkalis. Reasoning to the answer: Electrical conductivity depends directly on the concentration of mobile ions in solution; sulfuric acid is a strong diprotic acid yielding three ions per molecule (2H⁺ + SO₄²⁻), outperforming sodium hydroxide which produces only two ions, weak carbonic acid, and weak salt electrolytes. Common mistake: Equating strong bases with highest conductivity without considering the stoichiometric number of ions produced per mole.

Distractor check: A student might select options A, B, or D by confusing oxidation processes at the anode with the reduction reactions occurring at the cathode. Reasoning to the answer: During electrolysis, the cathode acts as the negative electrode where positively charged cations migrate, gain electrons in a reduction process, and are discharged as neutral atoms, such as Cu²⁺ ions turning into copper metal. Common mistake: Mixing up anode and cathode functions by attributing oxidation and anion discharge to the negative terminal.

Distractor check: A student might pick options A, C, or D due to decimal placement errors or confusion over volume units when converting cubic centimeters to cubic decimeters. Reasoning to the answer: First, convert the volume to decimeters by dividing 100.0 cm³ by 1000 to get 0.100 dm³, then multiply by the molarity of 0.20 mol/dm³ to find the moles as 0.020 mol, and finally multiply by the molar mass of NaOH (40 g/mol) to obtain 0.80 g. Common mistake: Forgetting to convert volume from cubic centimeters to cubic decimeters before calculating the number of moles.

Distractor check: A student might select options B, C, or D by confusing directional charge separation with metallic bonding or complete ionic transfer. Reasoning to the answer: Bonding between hydrogen and a highly electronegative partner like oxygen, nitrogen, or fluorine forces unequal sharing of bonding electrons, generating partial electrical charges that establish bond polarity. Common mistake: Confusing an internal bond's polarity with intermolecular dipole-dipole attractions.

Distractor check: A student might choose options A, B, or C, forgetting that asymmetric molecular shapes or electronegativity differences create polar bonds. Reasoning to the answer: Chlorine gas (Cl₂) consists of two identical chlorine atoms with identical electronegativity values, ensuring symmetrical and equal sharing of electrons that results in a strictly non-polar covalent bond. Common mistake: Assuming all covalent molecules are automatically non-polar regardless of atom symmetry.

Distractor check: A student might mistakenly pick option A, C, or D by incorrectly assigning potassium or oxygen oxidation numbers. Reasoning to the answer: In potassium dichromate (K₂Cr₂O₇), potassium has a fixed oxidation state of +1 and oxygen is -2, so setting up the equation 2(+1) + 2x + 7(-2) = 0 simplifies to 2 + 2x - 14 = 0, which yields 2x = 12 and therefore x = +6 for chromium. Common mistake: Forgetting to multiply the subscript numbers of atoms by their respective oxidation states when setting up the algebraic balance.

Distractor check: A student might select options B, C, or D by multiplying the relative molar mass instead of dividing it by the reference standard. Reasoning to the answer: Vapour density is defined as the ratio of the relative molar mass of a gas to that of hydrogen, which is calculated by dividing the molar mass by 2, yielding 30 divided by 2, or 15. Common mistake: Multiplying the molar mass by two instead of dividing when calculating vapour density.

Distractor check: A student might mistakenly choose options A, C, or D by misinterpreting the positive sign of the enthalpy change as a release of thermal energy. Reasoning to the answer: A positive enthalpy change value of +198 kJ/mol indicates that the chemical process absorbs thermal energy from its environment, classifying it strictly as an endothermic reaction rather than an exothermic one. Common mistake: Assuming any large numerical enthalpy value means energy is given off to the surroundings.

Distractor check: A student might pick evaporation, diffusion, or melting, confusing physical phase changes with chemical bond formation. Reasoning to the answer: The kinetic theory of matter explains particle motion, which accounts for physical phenomena like diffusion, evaporation, and melting where intermolecular forces are overcome by motion. Polymerization, however, involves chemical bond formation and molecular joining rather than particle motion evidence. Common mistake: Confusing chemical synthesis and polymerization processes with physical kinetic particle behaviors.

Distractor check: A student might choose options A, B, or C by confusing electrostatic attraction in ionic lattices with electron-sea models. Reasoning to the answer: Metallic bonding is characterized by positively charged metal ions immersed in a mobile sea of delocalized electrons, which accounts for properties like high electrical conductivity and malleability. Common mistake: Confusing the delocalized electron sea model of metals with localized electron sharing in covalent or dative bonds.