WAEC Chemistry 2019
Questions & Answers

Students often confuse atomic number with mass number when looking at periodic organization. The modern periodic table arranges elements strictly by increasing atomic number, which correlates to the proton count and drives periodic trends.

Covalent compounds might be guessed by students confusing molecular and ionic properties. Electrovalent compounds feature strong electrostatic forces between ions, which demand high energy amounts to melt or boil.

Potassium or silver might be picked if reactivity series rules are mixed up. Iron reacts very slowly with cold water to generate iron(II) hydroxide and hydrogen, whereas calcium reacts vigorously and silver shows no reaction.

Magnesium or calcium could be mistakenly chosen by confusing periodic metal trends with non-metals. Fluorine stands out as the most electronegative non-metal in the highest period, possessing the highest ionization energy due to intense nuclear attraction on its small valence electrons.

Hydration or reduction might seem chemically plausible to confused test-takers. Chlorine bleaches through oxidation by releasing nascent oxygen that breaks down colored compounds into colorless products.

Students might miscalculate by forgetting to multiply by the molar mass ratio or stoichiometry coefficient. Moles of Mg equal 6.5 divided by 24 (0.271 mol); since the equation shows 1 mol of Mg produces 1 mol of H2, multiplying 0.271 by 2 gives 0.542 g, which rounds to 0.54 g.

Being metals or forming cations describes many elements outside their specific group. Calcium and magnesium belong to the same group because both reside in Group 2 (alkaline earth metals) and share the exact same number of valence electrons.

Students might mistakenly select polar options like NH₃, H₂O, or HBr because they contain electronegativity differences and permanent dipoles. By contrast, Br₂ consists of identical atoms sharing electrons equally, eliminating any dipole moment and ensuring neutrality of charge distribution despite the other options possessing asymmetric shapes and polar bonds. Common mistake: confusing molecular symmetry and identical atom bonding with the presence of polar covalent bonds.

One might select other pairs containing ethene or ethyne due to visual similarities in formulas, but molar calculations reveal the exact match. Carbon (II) oxide has a mass of 12 + 16 = 28, and ethene (C₂H₄) totals (2 × 12) + (4 × 1) = 24 + 4 = 28, meaning both components share an identical molecular mass of 28. Common mistake: miscalculating atomic weights or omitting the number of atoms when summing masses.

A student might incorrectly choose concentrated or dilute because these describe the overall quantity of solute dissolved rather than the physical state condition. When a solution holds the absolute maximum amount of solute possible at a given temperature, a dynamic equilibrium is continuously maintained between the dissolution and crystallization processes, defining it as saturated. Common mistake: confusing the total amount of dissolved substance with the equilibrium state of excess solute.

An incorrect option like 0.20 or 0.50 mol might be picked if the molar mass ratio is miscalculated. Carbon (IV) oxide has a total molar mass of 44 g/mol, which includes 32 g of oxygen. Because the sample contains 16.0 g of oxygen, you divide that mass by the oxygen mass in one mole (16 / 32), yielding 0.50 mol of CO₂. Common mistake: dividing by the entire molar mass of CO₂ instead of focusing on the oxygen component mass.

A student might mistakenly choose the law of multiple proportion or constant composition because they relate to compounds and masses. However, stoichiometry relies entirely on Lavoisier's fundamental principle that mass is neither created nor destroyed, meaning mass is conserved during chemical reactions to calculate accurate reactant and product ratios. Common mistake: mixing up general laws of chemical combination with the conservation principle underlying stoichiometric balancing.

Options like 1.7% or 5.0% might be selected through arithmetic errors when determining individual element contributions. The entire molar mass of CH₃COOH equals 60 g/mol, while the combined mass of the four hydrogen atoms contributes 4 g/mol; calculating the percentage gives (4 / 60) × 100%, which equals 6.67%, rounding to 6.7%. Common mistake: failing to count all hydrogen atoms present in the molecular formula before calculating mass percentages.

One might mistakenly pick MgCl₂, NaCl, or AgCl because they are well-known metal chlorides, but they feature purely ionic bonding. Aluminum chloride instead forms dimers (Al₂Cl₆) through coordinate (dative) bonds, where chlorine atoms donate their lone pairs to aluminum atoms. Common mistake: assuming all metal halides share the same ionic bonding characteristics without considering dimerization.

A student could mistakenly choose a butane derivative because the main chain can be miscounted from the wrong direction. Finding the longest continuous carbon chain yields 5 carbons, making it a pentane derivative, and numbering from the end closest to the branch places the methyl group on the second carbon, resulting in 2-methyl pentane. Common mistake: failing to find the longest carbon chain and misidentifying the base alkane name.

Options such as high degree of purity or readily available might be mistakenly selected, but those are indeed required properties. A primary standard should possess a low molar mass for precise weighing along with high purity and ready availability; having a high molecular mass is actually undesirable. Common mistake: assuming larger molecular weights are better for analytical precision in titrations.

A student might choose 0.10 or 2.80 mol dm⁻³ through incorrect decimal placement or failing to convert volume to dm³. First, find the moles of X by dividing 0.28 g by 56 g, giving 0.005 mol; then, divide this by the volume in cubic decimetres (0.1 dm³), resulting in a solubility of 0.05 mol dm⁻³. Common mistake: omitting the conversion of solution volume from cubic centimeters to cubic decimeters.

Learners might select hydrochloric or hydrofluoric acid due to familiarity with strong laboratory acids. Vinegar is instead an aqueous solution containing 4-8% ethanoic acid, which is produced commercially through the fermentation of ethanol. Common mistake: confusing common household organic acids with strong mineral acids.

One might select ethanoic acid because it is an acid, but it is a weak electrolyte. Hydrochloric acid is a strong acid that undergoes complete dissociation in solution, producing a high hydrogen ion concentration ([H⁺] = 1 mol dm⁻³) and dropping the pH down to 0, which is the lowest among the choices. Common mistake: failing to distinguish between strong and weak acids when comparing pH levels of equimolar solutions.

A student could mistakenly select carbon (II) oxide or other neutral gases because they are common air pollutants. When carbon (IV) oxide dissolves in water, it reacts to form carbonic acid (H₂CO₃), which undergoes slight dissociation to increase hydrogen ions and lower the pH to an acidic level of approximately 6. Common mistake: assuming all atmospheric gases react with water to form acidic solutions.

Indicators like methyl orange or methyl red might be chosen because they are common acid-base indicators, but they change color at lower pH ranges. The titration between a weak acid (HCOOH) and a strong base (NaOH) produces an alkaline equivalence point around pH 8-9, making phenolphthalein—which changes color between 8.2 and 10.0—the most suitable choice. Common mistake: matching an indicator's pH range to the wrong combination of acid and base strengths.

Options like 0.5 or 3.0 might be selected by misreading the stoichiometric coefficients in the balanced equation. The reaction shows that 3 moles of H₂SO₄ are required to produce 1 mole of Al₂(SO₄)₃, meaning that to yield 0.5 mol of the product, you need half that amount of acid, which is 1.5 moles. Common mistake: failing to scale the molar coefficients proportionally based on the target product yield.

A student might pick strong acid and strong base, forgetting that weak components introduce extra energy steps. When a weak acid reacts with a strong base, the process includes endothermic ionization energy, which combines with the neutralization heat to produce a higher overall magnitude of enthalpy change (|ΔH|) than the standard ~57 kJ/mol of strong-strong neutralizations. Common mistake: assuming strong acid-strong base pairs always produce the maximum possible heat of neutralization.

One might guess that heat is absorbed if confusing hydration with endothermic dissolution processes. When quicklime (calcium oxide) dissolves and reacts with water to form slaked lime, the reaction is strongly exothermic, meaning it evolves heat. Common mistake: assuming all dissolution processes in water absorb heat from their surroundings.

A student could mistakenly choose exothermic or spontaneous by confusing thermodynamic sign conventions with reaction rates or favorability. A positive value for enthalpy change (ΔH) signifies that the system absorbs heat from its environment, which is the defining characteristic of an endothermic reaction. Common mistake: confusing a positive enthalpy change with heat being released by the reaction.

Options like CH₃OH or incorrect molecular formulas might be chosen through confusion over oxidation levels of alcohols. Ethanol (CH₃CH₂OH) undergoes oxidation in the presence of strong oxidizing agents to form ethanoic acid (CH₃CO₂H). Common mistake: misidentifying the carboxylic acid product resulting from primary alcohol oxidation.

A student might select the pop sound or lime water options by confusing this reaction with tests for hydrogen gas or carbon dioxide. Adding dilute sulfuric acid to iron (II) sulphide liberates hydrogen sulfide gas (H₂S), which reacts with lead (II) ethanoate paper to turn it black due to the formation of lead sulfide. Common mistake: confusing the gas produced by metal-acid reactions with sulfide-acid displacement products.

Options like CH₂O or C₂H₄O₂ might be picked if percentage calculations are incorrectly rounded. Assuming a 100 g sample gives 5.17 moles of carbon, 12 moles of hydrogen, and 1.625 moles of oxygen; dividing by the smallest value yields a ratio of roughly 3:7:1, which closely fits the empirical formula C₃H₆O. Common mistake: rounding mole ratios prematurely before finding the simplest whole-number empirical formula.

Students might choose 1,2-dichloroethene by assuming an addition reaction takes place, ignoring the saturated nature of the reactant. When excess chlorine is mixed with ethane, free radical substitution occurs, leading to sequential halogenation where 1,2-dichloroethane is formed as a primary product. Common mistake: treating alkanes as unsaturated hydrocarbons that undergo addition rather than substitution reactions.

Reagents like H⁺/K₂Cr₂O₇ might be chosen, but both primary and secondary alcohols react with them, making them unsuitable for distinction. The I₂/OH⁻ iodoform test specifically targets secondary alcohols like propan-2-ol to form a yellow precipitate, while primary alcohols like propan-1-ol do not give this positive result. Common mistake: using general oxidizing agents that fail to distinguish between different classes of alcohols.

A student might mistakenly think a catalyst affects the activation energy or reaction rates at equilibrium, but those are precisely what it alters. Adding a catalyst lowers the activation energy and increases the forward and reverse rates equally, leaving the heat of reaction (ΔH) entirely unaffected. Common mistake: believing a catalyst changes the overall thermodynamic energy profile or enthalpy of a reaction.

Options like an oxidizing agent or drying agent might be selected by misinterpreting the direction of electron transfer. Ammonia donates electrons and causes chlorine to be reduced to chloride ions, while the nitrogen within ammonia is itself oxidized from a -3 oxidation state to 0 in nitrogen gas, meaning ammonia functions as a reducing agent. Common mistake: confusing the substance being oxidized with the role it plays as a reducing agent.

Distractor check: A student might mistakenly choose option A (+4) or B (+5) by miscalculating the oxidation states or improperly balancing the dichromate ionic charge. Reasoning to the answer: To determine the oxidation number of chromium in the dichromate ion, set up the algebraic equation where the sum of all oxidation states equals the overall charge of -2. With oxygen fixed at -2, substitute the values so that 2x + 7(-2) = -2, which simplifies to 2x - 14 = -2, yielding 2x = 12 and therefore x = +6. Common mistake: Forgetting to equate the sum of the oxidation numbers to the overall ionic charge of -2.

Distractor check: Students could easily select saponification (A) or carboxylation (C) due to confusion regarding carboxylic acid derivatives and general organic reaction classifications. Reasoning to the answer: An alkanoic acid reacts reversibly with an alkanol under acid catalysis to produce an ester and water, represented by the equation RCOOH + R'OH ⇌ RCOOR' + H₂O. This fundamental reversible condensation process is formally known as esterification. Common mistake: Confusing esterification with saponification, which is the alkaline hydrolysis of fats.

Distractor check: A student might pick option B (be replaced by Al) by misinterpreting standard electrode potentials or confusing reducing agent strengths. Reasoning to the answer: Magnesium possesses a higher oxidation potential of +2.37 volts compared to aluminum's +1.66 volts, making magnesium a stronger reducing agent. Consequently, magnesium will readily displace aluminum ions from solution rather than being replaced itself. Common mistake: Assuming the metal with the lower oxidation potential is the more active reducing agent.

Distractor check: A student might select option D (shift the equilibrium position to the right) by misunderstanding how pressure changes affect gas phase equilibria. Reasoning to the answer: According to Le Chatelier's principle, lowering the pressure of a gaseous system causes the equilibrium to shift toward the side with a greater number of moles. For the given equation, the left side has 3 total moles of gas while the right side has only 2 moles, meaning a pressure reduction forces the equilibrium position to the left. Common mistake: Mixing up the direction of equilibrium shift when pressure is decreased versus when it is increased.

Distractor check: A student might choose option C (total pressure) by failing to distinguish between the individual contribution of a gas and the sum of all gases in a mixture. Reasoning to the answer: In a mixture of chemically non-reacting gases, each individual gas exerts a pressure independently of the others, which is known as its partial pressure according to Dalton's law. Common mistake: Confusing partial pressure with total system pressure.

Distractor check: Students often guess water treatment (B) because wastewater and municipal water processing sound similar. Reasoning to the answer: Biotechnology utilizes specific microbes and enzymes for biological wastewater treatment, breaking down organic pollutants through both aerobic and anaerobic processes in sewage management. Common mistake: Assuming biological treatment applies to raw municipal drinking water rather than sewage.

Distractor check: A student might choose one of the heavier straight-chain alkanes like C₆H₁₄ (A) by confusing molecular mass trends with branching effects. Reasoning to the answer: Boiling point generally increases as molecular size and mass increase. Among the listed options, butane (C₄H₁₀) has the lowest molecular mass of 58 g/mol, giving it the lowest boiling point compared to the larger pentane and hexane chains. Common mistake: Overlooking the direct relationship between carbon chain length and boiling point.

Distractor check: Option A (ethane and ethene) is a common trap for students who mix up the saturation levels of the hydrocarbon reactants and products. Reasoning to the answer: The chemical transformation involves the partial hydrogenation of ethyne (C₂H₂) utilizing a hydrogen catalyst to form ethene (C₂H₄), meaning the starting material is ethyne and the resulting product is ethene. Common mistake: Misidentifying unsaturated alkynes and alkenes during hydrogenation reactions.