WAEC Chemistry 2018
Questions & Answers

A student might mistakenly choose methane or hydrogen thinking of general petrochemical fuels rather than specific polymerization monomers. To identify the correct raw material, recognize that ethene molecules undergo addition polymerization to link together and form polyethene, which is a foundational polymer utilized heavily throughout the plastic manufacturing sector. Common mistake: Confusing simple alkanes like methane with unsaturated alkenes like ethene used in polymer synthesis.

A test-taker could mistakenly select saturated alkanes like pentane, propane, or hexane, forgetting that single bonds resist addition pathways. To determine the correct hydrocarbon, evaluate the delocalized pi electron ring system of aromatic compounds which permits them to undergo electrophilic substitution as well as hydrogenation addition under specific catalytic conditions. Common mistake: Assuming all hydrocarbons can easily undergo addition reactions regardless of saturation.

A student might mistakenly choose the neutralization or precipitation options if they confuse ionic double-displacement with electron transfer processes. To find the correct redox reaction, track the oxidation states of each atom to find where species undergo simultaneous loss and gain of electrons, such as sulfur changing its oxidation number from negative two and positive four to zero. Common mistake: Mistaking simple acid-base neutralization reactions for oxidation-reduction processes.

A student might mistakenly choose oxidation because the process involves chemical furnaces and high-temperature gas interactions. To isolate the correct chemical operation, trace the removal of oxygen from iron oxide using carbon monoxide where iron ions gain electrons to form pure metal, which is the definition of reduction. Common mistake: Confusing the oxidation of the reducing agent carbon monoxide with the reduction of the metal ore itself.

One might mistakenly select 80.0 g dm minus three by multiplying the volume incorrectly instead of scaling up to a full liter. To solve for solubility, divide the mass of salt obtained by the volume of the evaporated solution in cubic decimeters, scaling the measurement proportionally to find the concentration present in one full cubic decimeter. Common mistake: Failing to convert cubic centimeters into cubic decimeters before calculating solubility.

A student might mistakenly choose hydrogen atoms in solution because the term 'acidic' immediately brings hydrogen to mind. To determine the true nature of an acidic salt, look at how compounds containing ionizable hydrogen dissociate in water to directly release hydrogen ions into the aqueous environment. Common mistake: Confusing the mere presence of bonded hydrogen atoms with the release of free hydrogen ions in solution.

A student might mistakenly select a negative enthalpy change or energy relationships where bond forming exceeds bond breaking because they confuse endothermic with exothermic properties. To identify an endothermic process, observe that thermal energy is absorbed from the immediate surroundings, which causes the exterior of the reaction vessel to feel noticeably cool to the touch. Common mistake: Confusing the temperature drop of the surroundings with heat release by the system.

A student might mistakenly select covalent hydrides like methane or ammonia because hydrogen is a nonmetal. To identify the ionic exception, look at metal hydrides where hydrogen accepts an electron to form a negatively charged hydride ion that electrostatically bonds with an alkali metal cation. Common mistake: Assuming hydrogen always forms covalent bonds when combining with other elements.

A test-taker might mistakenly pick a reducing agent because they confuse the role of the halogen being reduced with the substance it acts upon. To determine the correct behavior, observe that the bromine molecule accepts electrons from iodide ions to convert them into iodine, thereby causing oxidation and acting itself as an oxidizing agent. Common mistake: Confusing the agent that undergoes reduction with the substance it oxidizes.

A student might select an incorrect formula like C two H four by improperly dividing the molar mass. To find the correct molecular formula, divide the given molar mass by the mass of the empirical unit to find the scaling integer, then multiply each subscript in the empirical formula by that integer. Common mistake: Using the empirical formula mass directly as the molecular mass without scaling by the integer multiplier.

Someone might mistakenly choose hydrolysis or decomposition by misinterpreting how organic functional groups break apart. To find the correct chemical pathway, examine the removal of a water molecule from ethanol using concentrated sulfuric acid, which strips the hydroxyl group and adjacent hydrogen to form a double bond in ethene. Common mistake: Confusing hydration additions with the acid-catalyzed dehydration of alcohols.

A student might mistakenly choose a decrease in pressure by miscounting gas moles or selecting a catalyst that changes equilibrium position. To determine what favors the backward direction in an endothermic equilibrium, apply Le Chatelier's principle by noting that lowering the temperature forces the system to shift in the exothermic direction to generate heat. Common mistake: Forgetting that temperature changes affect endothermic and exothermic directions oppositely.

A student might mistakenly select 0.049 g by miscalculating the volume conversion or omitting the molar mass factor entirely. To arrive at the correct mass, first multiply the molar concentration by the volume in cubic decimeters to find the total moles, and then multiply that mole quantity by the calculated molar mass of sulfuric acid. Common mistake: Forgetting to convert the volume from cubic centimeters to cubic decimeters.

A student might select reducing the quantity of water, thinking smaller volumes boil faster regardless of pressure conditions. To determine the correct physical method, recall that boiling occurs when vapor pressure equals external pressure, meaning that dropping the ambient pressure allows vapor pressure to match it at a lower temperature. Common mistake: Confusing the time required to reach boiling temperature with the actual boiling point temperature itself.

A student might mistakenly select potassium nitrate or potassium chloride by guessing based on general salt familiarity. To identify the salt with the highest solubility among the given choices at twenty degrees Celsius, compare their tabulated gram saturation limits per one hundred grams of water where sodium chloride reaches the highest value. Common mistake: Assuming all common laboratory salts share identical solubility curves at room temperature.

A student might select supersaturation or assume the salt dissolves completely by ignoring the definition of saturation limits. To determine the correct outcome, recognize that a saturated solution already holds the maximum possible solute, so any extra salt added simply remains undissolved at the bottom while dynamic equilibrium is maintained. Common mistake: Assuming adding more solute to a saturated solution will always cause it to dissolve.

A student might mistakenly choose propane or an alcohol because they overlook functional group reactivities toward halogens. To identify the correct compound, look for unsaturation in alkenes like propene, which readily undergo rapid addition reactions with bromine water to yield a colorless dibromoalkane product. Common mistake: Confusing saturated alkanes with unsaturated alkenes when testing with halogen water.

A student might select electrolytic cell types or mechanisms instead of the material properties at the interface. To determine what governs preferential ion discharge, consider how the chemical nature of the electrode—whether inert like platinum or active like copper—can alter the standard reduction potentials and dictate which ions react. Common mistake: Overlooking the physical and chemical influence of electrode materials during preferential ion selection.

A student might incorrectly select options like the 2s or 2p orbitals by assuming electrons fill shells strictly sequentially without regard to energy levels or principal quantum numbers. To arrive at the correct orbital, consider that magnesium possesses a total atomic number of 12, distributing its electrons into the electronic configuration 1s² 2s² 2p⁶ 3s². This arrangement places the outermost electrons in the 3s orbital. Common mistake: Confusing the highest principal energy level with inner core orbitals when locating valence electrons.

One might mistakenly pick options containing only iron and carbon, such as plain carbon steel, or add aluminum incorrectly, overlooking the specific formulation required for specialized properties. Stainless steel is fundamentally an alloy comprising iron combined with carbon and nickel, where nickel and carbon are introduced to impart enhanced strength and corrosion resistance. Common mistake: Forgetting that stainless steel requires nickel alongside iron and carbon for its corrosion-resistant qualities.