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.