WAEC Chemistry 2024
Questions & Answers

Students might mistakenly pick oxygen or hydrogen, which are lighter than air and tend to escape upward. Chlorine is denser than air because its molecular mass of 71 is greater than the average molecular mass of air, allowing it to displace air downward. Common mistake: confusing the densities of gases relative to air.

Substances like water or aqueous sodium chloride might be incorrectly chosen due to unfamiliarity with intermolecular bonding types. Tetrachloromethane has the lowest boiling point because it relies solely on weak van der Waals forces, whereas ethanol and water exhibit stronger hydrogen bonding. Common mistake: underestimating the strength of hydrogen bonding compared to van der Waals forces.

One might mistakenly select physics or biology by confusing the study of matter with the study of energy or living organisms. Chemistry deals specifically with the nature, properties, and conversions of substances, distinguishing it from biology and physics. Common mistake: confusing the definitions of chemistry and physics.

A student might incorrectly choose Hund's rule or Pauli's exclusion principle by confusing the various quantum rules. This configuration contravenes Aufbau's principle if applied to sodium, because it fills the 2p orbital before the 3s orbital which should be filled first. Common mistake: confusing Aufbau's principle with Hund's rule.

An option like +2 or +7 might be mistakenly selected by failing to account for the subscripts of the atoms in the formula. Setting the sum of the oxidation numbers in Na2Cr2O7 to zero using sodium at +1 and oxygen at -2 yields an equation where 2x equals 12, making chromium +6. Common mistake: forgetting to multiply individual oxidation states by their respective subscripts.

Substances like haematite or magnetite might be chosen incorrectly because they are also prominent metal ores. Bauxite is recognized as the most important ore of aluminium, while haematite and magnetite are primary ores of iron. Common mistake: confusing aluminum ores with iron ores.

Options involving acids, such as turning litmus paper from blue to red, might be mistakenly selected. Bases turn red litmus paper blue due to the presence of hydroxide ions, whereas acids exhibit the reverse color change. Common mistake: mixing up the color change directions for acids and bases on litmus paper.

A student might mistakenly pick an alkene or ether like ethene or dimethyl ether by misinterpreting oxidation pathways. Excess acidified potassium dichromate completely oxidizes the primary alcohol ethanol into the carboxylic acid ethanoic acid. Common mistake: stopping the oxidation at the aldehyde stage when excess oxidizing agent is present.

Options like 60 g/mol or 80 g/mol could be chosen by omitting an atomic mass or miscalculating the multiplication. Summing the individual atomic masses of calcium, carbon, and three oxygen atoms gives 40 plus 12 plus 48, totaling 100 g/mol. Common mistake: failing to multiply the oxygen atomic mass by its subscript of three.

A student might mistakenly choose a pH greater than 7 by assuming all salts dissolve to form basic solutions. Ammonium salts dissolve in water to form hydronium ions, producing an acidic solution with a pH less than 7. Common mistake: assuming ammonium compounds yield basic rather than acidic solutions.

Techniques like filtration or distillation might be incorrectly chosen because they are also common laboratory separation methods. Ethanedioic acid is an organic solid that is best purified through crystallization using a suitable solvent. Common mistake: confusing distillation for liquids with crystallization for soluble solids.

Functional groups like -OH or -CHO might be mistakenly selected by confusing acids with alcohols or aldehydes. The carboxyl group, represented as -COOH, is the defining functional group present in all alkanoic acids. Common mistake: confusing the carboxylic acid group with the aldehyde or alcohol functional groups.

A student might incorrectly select covalent bonds only by assuming intermolecular forces are the same as intramolecular bonds. Ethanol features both hydrogen bonding from its hydroxyl group and van der Waals forces along its hydrocarbon chain. Common mistake: ignoring van der Waals forces in molecules that also possess hydrogen bonding.

Options like 32.7% or 4.08% might be chosen by miscalculating the total molar mass or using the wrong mass for hydrogen. Dividing the total mass of hydrogen by the molar mass of sulfuric acid and multiplying by 100 yields approximately 2.04%. Common mistake: forgetting to multiply the atomic mass of hydrogen by its subscript of two.

1) Distractor check: Students might erroneously choose option A, B, or D by confusing oxidation states with oxidizing agents or reversing which species loses and gains electrons. 2) Reasoning to the answer: In this transformation, Ni²⁺ gains electrons and undergoes reduction, which makes it function as the oxidizing agent, whereas solid iron loses electrons and undergoes oxidation, designating it as the reducing agent. 3) Common mistake: Reversing the roles of the oxidizing and reducing agents based solely on ion charges.

1) Distractor check: Students might select option A because elements near the transition series can be reactive, or option B assuming metals always have low melting temperatures. 2) Reasoning to the answer: Transition elements feature partially filled d-orbitals that allow them to exhibit multiple oxidation states. They tend to have high melting points and form colored compounds while being less reactive than alkali metals. 3) Common mistake: Assuming transition metals share the high reactivity characteristic of Group 1 alkali metals.

1) Distractor check: Students might pick option A due to confusing aircraft requirements with brittle materials, or option C thinking high density is desirable for flight. 2) Reasoning to the answer: Aluminium is exceptionally well-suited for aviation because it combines a low density of 2.7 g/cm³ for lightweight structures with a protective oxide layer that grants high resistance to corrosion. 3) Common mistake: Forgetting that aircraft require lightweight materials with low density rather than high density.

1) Distractor check: Students might choose option B or D by confusing periodic trends or missing the general left-to-right increase across a period. 2) Reasoning to the answer: Ionization energy rises steadily across a period due to a higher effective nuclear charge, proceeding from Al in group 13 to Si in group 14, P in group 15, and S in group 16. 3) Common mistake: Failing to account for group 15 stability anomalies or general periodic trends when ordering elements.

1) Distractor check: Students might select option B or D by misapplying the stoichiometric mole ratio or omitting the molar volume multiplication step. 2) Reasoning to the answer: According to the balanced chemical equation, 1 mole of C₃H₈ yields 3 moles of CO₂ gas, meaning 0.1 mole produces 0.3 mole of CO₂. Multiplying this result by the molar volume at stp yields 0.3 multiplied by 22.4 dm³, resulting in 6.72 dm³. 3) Common mistake: Forgetting to multiply the calculated moles of the gas by the molar volume at stp.

1) Distractor check: Students might pick option A or D by taking a simple arithmetic mean instead of a weighted average of the isotope masses. 2) Reasoning to the answer: The relative atomic mass is determined by calculating the weighted average using the respective percentages: (10 times 0.199) plus (11 times 0.801), which gives 1.99 plus 8.811, totaling 10.801, or approximately 10.8. 3) Common mistake: Calculating a straightforward unweighted average of the isotope mass numbers instead of using percentage abundances.

1) Distractor check: Students might choose option C or D by counting all valence electrons instead of focusing strictly on the shared electron pairs. 2) Reasoning to the answer: Methane contains four carbon-hydrogen single bonds where each bond involves a shared pair of electrons, bringing the total number of shared pairs to four. 3) Common mistake: Counting the total individual electrons involved in bonding rather than the number of electron pairs.

1) Distractor check: Students might choose option C by mistakenly thinking that neutron count dictates the chemical reactivity of an element. 2) Reasoning to the answer: Isotopes share the same atomic number and identical proton counts, which dictates their electron configuration and ensures their chemical behavior remains similar despite varying neutron numbers. 3) Common mistake: Assuming that differences in neutron count alter the chemical properties of an element.

1) Distractor check: Students might pick option B or D because they are liquids, or option A because it is neutral, ignoring how excess stomach acid requires neutralization. 2) Reasoning to the answer: Excess stomach acid, which consists of hydrochloric acid, requires a basic substance with a pH above 7 to neutralize it, making milk of magnesia with a pH of 10.5 the appropriate choice. 3) Common mistake: Selecting acidic or neutral household liquids when attempting to neutralize excess gastric acid.

1) Distractor check: Students might choose option B, C, or D through arithmetic errors when summing the total mass of oxygen atoms or dividing by the molar mass. 2) Reasoning to the answer: The compound contains 11 oxygen atoms in total (4 from the sulfate group and 7 from the water of crystallization), giving a total mass of 176 for oxygen. Dividing 176 by the molar mass of 246 and multiplying by 100 gives approximately 71.5%. 3) Common mistake: Forgetting to include the oxygen atoms contributed by the water molecules in the hydrated salt.

1) Distractor check: Students might select option B or C by confusing the mechanism of a catalyst with changing overall reaction energy or physical reactant properties. 2) Reasoning to the answer: Catalysts facilitate faster reaction rates by offering an alternative, lower-energy pathway that decreases the activation energy without being consumed in the process. 3) Common mistake: Believing that catalysts alter the net energy change of a reaction rather than providing an alternative pathway.

1) Distractor check: Students might choose option A or B by misreading the stoichiometric coefficients or failing to scale the volume ratio correctly. 3) Reasoning to the answer: The balanced equation indicates that 2 volumes of ethane produce 6 volumes of steam, meaning 400 cm³ of ethane will yield 6 divided by 2 multiplied by 400 cm³, resulting in 1200 cm³. 3) Common mistake: Using an incorrect mole or volume ratio from the balanced gaseous equation.

1) Distractor check: Students might select option A, B, or C thinking that heating a saturated solution promotes crystallization rather than cooling it. 3) Reasoning to the answer: Lowering the temperature reduces solute solubility, leading to supersaturation and causing crystals to form at a colder temperature such as 20°C. 3) Common mistake: Assuming that raising the temperature induces crystallization in a saturated solution.

1) Distractor check: Students might choose option B or C by misidentifying biological macromolecules and assuming all natural organic compounds are polymers. 3) Reasoning to the answer: Fats are classified as simple esters formed from glycerol and fatty acids rather than large macromolecules, whereas rubber, starch, and proteins are true polymers. 3) Common mistake: Grouping fats into the polymer category alongside polysaccharides and polypeptides.

1) Distractor check: Students might select option B, C, or D because they represent valid tenets of collision theory while overlooking the false statement in option A. 3) Reasoning to the answer: Collision theory dictates that molecules must collide with sufficient energy and proper orientation, but elastic or inelastic collisions can involve energy transformations as long as the activation energy is met, meaning coming into contact without energy loss is not a strict requirement. 3) Common mistake: Assuming all molecular collisions must occur with zero loss of kinetic energy.

1) Distractor check: Students might choose option A or C by confusing weak acidity with corrosiveness or salt formation capacity. 3) Reasoning to the answer: Weak acids, such as ethanoic acid, undergo only partial dissociation in aqueous solution, resulting in low electrical conductivity and a pH closer to neutral than strong acids. 3) Common mistake: Concluding that weak acids do not ionize at all instead of recognizing that they ionize slightly.

1) Distractor check: Students might pick option A, C, or D by confusing esterification reagents with hydrolysis or saponification conditions. 3) Reasoning to the answer: The preparation of alkanoates from alkanols and alkanoic acids is an esterification process that requires concentrated sulfuric acid to act as a dehydrating agent alongside the application of heat. 3) Common mistake: Using aqueous sodium hydroxide instead of an acid catalyst and heat for ester synthesis.

1) Distractor check: Students might select option A or B by miscalculating the molar mass of carbon dioxide or incorrectly multiplying the mole value. 3) Reasoning to the answer: The molar mass of carbon dioxide is 44 g/mol, so 22 grams represents 0.5 mole. Multiplying this mole value by the stp molar volume of 22.4 dm³ gives 11.2 dm³. 3) Common mistake: Multiplying the mass directly by the molar volume without converting mass to moles first.

1) Distractor check: Students might pick option B, C, or D by confusing non-bonding valence electron pairs with bonding pairs or overall valence electrons. 3) Reasoning to the answer: Outermost shell electron pairs that do not participate in chemical bonding are designated as lone pairs, which play a crucial role in determining molecular geometry via VSEPR theory. 3) Common mistake: Confusing lone pairs with shared bonding pairs of electrons.

1) Distractor check: Students might choose option B or C by confusing isoelectronic species with isotopes or allotropes. 3) Reasoning to the answer: Species with the same total number of electrons despite having different atomic nuclei are described as isoelectronic. 3) Common mistake: Mixing up isoelectronic terms with isotopes which share the same proton count.

1) Distractor check: Students might choose option B or C by assuming water contaminants always preserve neutrality or make the solution acidic. 3) Reasoning to the answer: Quicklime reacts with water to form calcium hydroxide, which is a basic substance that raises the pH above 7, resulting in a mild alkalinity of approximately 8 for dilute solutions. 3) Common mistake: Forgetting that basic metal oxides react with water to form alkaline solutions.

1) Distractor check: Students might select option B by following the standard Aufbau filling order strictly and failing to account for d-subshell stability exceptions. 3) Reasoning to the answer: Copper achieves extra stability through a fully filled d-subshell, resulting in an electron configuration of [Ar] 4s¹ 3d¹⁰ rather than the expected 4s² 3d⁹ arrangement. 3) Common mistake: Forgetting the electron configuration exception for copper and chromium regarding half-filled or fully filled d-orbitals.

1) Distractor check: Students might pick option B or D by assuming all element combinations produce covalent bonds. 3) Reasoning to the answer: Metals in the s-block feature low ionization energies while non-metals in the p-block possess high electronegativities, causing them to undergo electron transfer and form electrovalent ionic bonds. 3) Common mistake: Assuming s and p block elements always share electrons covalently instead of transferring them.

1) Distractor check: Students might choose option D by confusing the slipperiness of graphite layers with overall material hardness. 3) Reasoning to the answer: Diamond exhibits extreme hardness due to its robust three-dimensional tetrahedral network held together by sp³ hybridized covalent bonds. 3) Common mistake: Confusing the softness of graphite with the extreme hardness of diamond despite both being allotropes of carbon.

1) Distractor check: Students might choose option C or D by referencing lone pairs or valence counts without considering their mobility within the crystal lattice. 3) Reasoning to the answer: Aluminium features a large number of delocalized valence electrons moving freely throughout its metallic lattice, creating strong electrostatic attractions that impart high mechanical strength and electrical conductivity. 3) Common mistake: Assuming valence electrons remain localized rather than delocalized in metallic bonding.

Distractor check: A student might mistakenly select option D (4) due to confusing the group number with the actual covalent bonding capacity. Reasoning to the answer: Nitrogen possesses five valence electrons in its outer shell. To achieve a stable octet configuration and fulfill its chemical stability requirements, it establishes three covalent bonds, as seen in ammonia (NH3), while retaining one unshared lone pair of electrons. Common mistake: Confusing total valence electrons with the number of bonds formed.