JAMB Chemistry 2008
Questions & Answers

Distractor check: A student might select A (acetic acid) due to confusing carbon chain lengths, B (propanal) due to confusing functional groups, or C (propanol) by mistaking the acid for an alcohol. Reasoning to the answer: The compound CH3CH2COOH contains three carbons linked to a carboxyl group (-COOH), identifying it as propanoic acid (C3H6O2), whereas acetic acid has two carbons, propanal is an aldehyde, and propanol is an alcohol. Common mistake: Confusing the carboxyl functional group of organic acids with the hydroxyl group of alcohols or the carbonyl group of aldehydes.

Distractor check: A student might pick A, assuming fats remain alongside acids, C, confusing the chemical process with food products, or D, mistaking the product for the starting material. Reasoning to the answer: Alkaline hydrolysis (saponification) of fats and oils with a base such as NaOH produces soap in the form of fatty acid salts along with glycerol. Common mistake: Forgetting that saponification breaks down ester linkages to yield glycerol alongside soap.

Distractor check: A student might mistakenly select A, B, or C, knowing that all listed options are indeed acids that share typical acidic properties like lowering pH, turning litmus red, and neutralizing alkalis. Reasoning to the answer: While HBr, HNO3, H2CO3, and H2SO4 have pH < 7, turn blue litmus red, and neutralize alkalis, HBr is a non-oxidizing acid that does not effectively react with marble (CaCO3) to liberate CO2 like the others do, making D incorrect. Common mistake: Assuming every strong or mineral acid shares identical reactivity toward carbonates and marble.

Distractor check: A student might pick B, N27.00, by directly correlating molar masses without calculating charges, or A and D through simple arithmetic errors in the faradaic ratio. Reasoning to the answer: Mg²⁺ requires 2 moles of electrons per 24 g, meaning the charge for 1 g Mg is (2 × 96500) / 24 = 8033.33 C, while Al³⁺ requires 3 moles of electrons per 27 g, giving a charge for 10 g Al of (3 × 96500 × 10) / 27 = 107222.22 C, which scales the cost to approximately N33.33. Common mistake: Forgetting to account for the valency (number of moles of electrons transferred) of each metal ion during Faraday calculations.

Distractor check: A student might pick B (10.60 g) by mistakenly doubling the expected charge or C and D through incorrect molar mass ratios. Reasoning to the answer: The reduction half-reaction Cu²⁺ + 2e⁻ → Cu shows that 1 mole of Cu (63.5 g) requires 2 × 96500 = 193000 C, so dividing 16000 C by 193000 C gives approximately 0.0829 moles, which yields a mass of about 5.30 g of copper. Common mistake: Failing to multiply Faraday's constant by the valency of copper (2) when determining the total charge required per mole.

Distractor check: A student might select A (RCOOR) for esters, B (RCHO) for aldehydes, or D (RCOR) for ketones by confusing the structural functional groups. Reasoning to the answer: Carboxylic acids are uniquely identified by the presence of the -COOH group, represented generally as RCOOH, whereas esters, aldehydes, and ketones contain distinct functional linkages. Common mistake: Confusing the carboxyl group (-COOH) with the carbonyl group (-CHO or -CO-).

Distractor check: A student might choose A, C, or D, knowing that alkenes undergo addition reactions, possess double bonds, and include ethene as a member. Reasoning to the answer: Alkenes are unsaturated hydrocarbons containing at least one double bond and undergo addition reactions with halogens, making B incorrect because alkenes are unsaturated, unlike saturated alkanes. Common mistake: Confusing saturated hydrocarbons (alkanes) with unsaturated hydrocarbons (alkenes).

Distractor check: A student might pick A, B, or C, believing that R is not a hydrogen isotope, U and Y are not isotopes, or that the elements listed are non-metals. Reasoning to the answer: R is tritium (³H, a hydrogen isotope), U and Y are potassium isotopes (¹⁹), and R, U, S (²⁴Cr), and T (²⁰Ca) are metals or metalloids, meaning T (calcium) is a metal rather than a noble gas, making D incorrect. Common mistake: Misidentifying calcium (atomic number 20) as a member of Group 18 (noble gases).

Distractor check: A student might select A, C, or D, confusing the reagent used for nitrogen with those meant for absorbing carbon dioxide, oxygen, or water vapor. Reasoning to the answer: Heated magnesium reacts directly with nitrogen to form magnesium nitride (Mg3N2), effectively removing the nitrogen gas from a nitrogen-oxygen mixture and leaving the oxygen behind. Common mistake: Confusing the action of heated magnesium (which removes nitrogen) with heated phosphorus (which removes oxygen).

A student might incorrectly select options A, B, or D because they expect properties like ice formation or temperature dependency to define water hardness. Hardness is determined by the presence of calcium and magnesium ions which react with soap to create an insoluble scum, making option C the correct choice. Common mistake: confusing water hardness with freezing points or general solvent behaviors.

Students could mistakenly select hygroscopic (B), efflorescent (C), or hydrated (D) by confusing how various substances interact with atmospheric moisture. Sodium hydroxide pellets are deliquescent because they capture moisture from the air and completely dissolve in it. Common mistake: failing to distinguish between hygroscopic substances that merely absorb moisture without dissolving and deliquescent ones that liquefy.

A test-taker might incorrectly pick A, B, or C thinking all options share the same chemical classification. Structures A, B, and C are isomers sharing the molecular formula C4H10O, whereas D represents a ketone with the formula C4H8O, meaning it differs in both functional group and formula. Common mistake: overlooking the difference between an alcohol/ether group and a carbonyl ketone group.

An examinee might select option A or C by confusing the general formulas assigned to alkenes or alkynes. Alkanes are saturated hydrocarbons containing only single bonds, and their formula is established as CnH2n+2 by matching saturated ratios like methane (CH4). Common mistake: mixing up alkane, alkene, and alkyne general formulas.

Students might mistakenly choose polymerization (A), substitution (C), or reduction (D) if they misinterpret how halogens react with hydrocarbons. A liquid hydrocarbon like an alkene reacts by adding across its double bond in the presence of sunlight, thereby consuming all the chlorine gas via an addition reaction. Common mistake: assuming all hydrocarbon-halogen reactions proceed through substitution regardless of saturation.

A student might incorrectly choose dehydrating agent (A) since sulfuric acid does remove water during the process. However, its primary chemical function in the esterification of ethanoic acid and ethanol is to protonate the carboxylic acid and act as a catalyst. Common mistake: confusing a reagent's secondary dehydrating ability with its primary catalytic role.

An examinee could incorrectly select options A, C, or D if they assume other metal salts generate carbon dioxide upon reacting with acids. Sea shells contain calcium carbonate, which reacts with dilute hydrochloric acid to liberate carbon dioxide gas that subsequently turns lime water milky. Common mistake: misidentifying which common salts release carbon dioxide when treated with an acid.

Students might mistakenly choose Ca²⁺ (B), Al³⁺ (C), or Pb²⁺ (D) by confusing the solubility profiles of various metal hydroxides in excess reagents. The zinc ion (Zn²⁺) forms a white precipitate of zinc hydroxide that dissolves in excess sodium hydroxide to form a tetrahydroxozincate complex and in excess ammonia to form an ammine complex. Common mistake: overlooking whether a hydroxide precipitate is amphoteric and soluble in excess ammonia.

An examinee might incorrectly choose B, C, or D by misidentifying the longest carbon chain or misnumbering the position of the substituent group. The longest continuous carbon chain contains four carbons making it a butane, and placing the methyl group on carbon two yields 2-methylbutane. Common mistake: numbering the carbon chain from the wrong end or miscounting the main chain length.

A student might select options A, C, or D by incorrectly performing the stoichiometry or molar volume conversions. Calculating the moles of sulfuric acid gives 0.25 mol, and since the acid reacts in a 1:1 mole ratio to produce hydrogen, multiplying by the molar gas volume of 22.4 dm³ results in 5.6 dm³. Common mistake: miscalculating the initial number of moles from concentration and volume.

An examinee could mistakenly pick sodium (A), magnesium (B), or calcium (D) by confusing main-group classifications across the periodic table. Chlorine has the outer electron configuration 3s²3p⁵ placing it in the p-block, whereas sodium, magnesium, and calcium are s-block elements. Common mistake: confusing s-block alkali and alkaline earth metals with p-block halogens.