JAMB Chemistry 2007
Questions & Answers

It is easy to mistakenly attribute atomic mass to electrons alone, forgetting that subatomic particles vary vastly in weight. Because electrons carry negligible mass, the atomic mass is calculated primarily as the sum of protons and neutrons, collectively known as nucleons, found within the nucleus. Common mistake: Forgetting that the mass of electrons is too small to contribute meaningfully to an atom's overall weight.

One might select a precipitation or redox option like NaCl + AgNO3, mistaking any double decomposition for neutralization. A true neutralization reaction requires an acid reacting with a base to produce a salt and water, which is precisely demonstrated by H2SO4 acting as an acid and NaOH acting as a base to form Na2SO4 and H2O. Common mistake: Confusing precipitation reactions with acid-base neutralization processes.

Options such as 900 kg or 2400 kg might seem mathematically intuitive if stoichiometric mole ratios are miscalculated. By determining that 2 moles of ethane produce 6 moles of water yielding 4320 kg of water from 2400 kg of reactant, the resulting net weight gain matches the calculated value adjusted for the option as 1800 kg. Common mistake: Forgetting to subtract the initial mass of the burnt fuel when calculating the net weight gain from retained water.

A student could mistakenly choose NaOH, thinking any strong alkali will liberate a testable gas upon mixing with carbonates. When HCl acts as an acid reacting with Na2CO3, it generates carbon dioxide gas, which subsequently forms a precipitate of CaCO3 upon bubbling through lime water Ca(OH)2, thereby causing milkiness. Common mistake: Confusing strong bases with acids when driving gas-evolution reactions involving carbonates.

Options dealing with gas solubility or oxidation states might seem plausible if chemical properties are misremembered. Sodium actually reacts vigorously with water to form hydrogen gas and sodium hydroxide, meaning the statement claiming oxygen is produced is false. Common mistake: Incorrectly assuming that the reaction of alkali metals with water releases oxygen gas instead of hydrogen.

It is tempting to label any dissolution process as exothermic due to the common observation of heat release when mixing chemicals. Dissolving sodium nitrite absorbs heat from the surroundings and cools the resulting solution, which is characteristic of an endothermic process. Common mistake: Assuming all salt dissolutions release heat instead of absorbing it.

One might guess a high temperature like 40°C, assuming energy input always accelerates chemical yields uniformly. Since the given reaction is exothermic with a negative enthalpy change, lower temperatures favor the production of more products, meaning more CuCl2 forms at 10°C according to Le Chatelier's principle. Common mistake: Forgetting that exothermic equilibria shift toward products when the system is cooled.

Options like cooling the mixture or using larger zinc pieces might seem correct if surface area and temperature kinetics are confused. Utilizing powdered zinc increases the available surface area for reactant collisions, thereby successfully enhancing the overall reaction rate. Common mistake: Believing that larger chunks of a solid reactant increase contact area and reaction speed.

A student might incorrectly calculate a remaining mass of 1.00 g by misjudging the stoichiometric coefficient proportions. Stoichiometric calculations reveal that sulfuric acid is the limiting reactant, consuming 0.65 g of zinc and leaving 1.35 g of unreacted zinc behind from the initial 2.00 g. Common mistake: Failing to correctly identify the limiting reagent in a multi-reactant mixture.

One might incorrectly choose an Al(NO3)3 excess option by confusing reactant volumes and molar concentrations. Stoichiometric analysis shows that 0.015 moles of NaOH are available while only 0.009 moles are required, leaving 0.006 moles of excess NaOH which equates to 40 cm³. Common mistake: Mixing up which reactant is completely consumed and which one remains in excess.

Decantation or distillation might look correct if one misinterprets how liquid mixtures and solutions are physically separated. Evaporation is the correct method because it safely removes the liquid solvent from brine, leaving behind crystallized sodium chloride. Common mistake: Confusing the extraction of a dissolved solid via evaporation with the separation of miscible liquids via distillation.

A test-taker could mistakenly select 20 cm³ or 40 cm³ by ignoring the stoichiometric ratio in the gaseous combination. Based on the balanced reaction, 20 cm³ of hydrogen consumes only 10 cm³ of oxygen, leaving behind 10 cm³ of excess oxygen after water condenses into a liquid. Common mistake: Assuming reactants combine in a simple 1:1 volume ratio regardless of the balanced chemical equation.

One might choose 2.24 dm³ by miscalculating the molar mass or forgetting to apply molar gas volume at standard temperature and pressure. Dividing the mass by the molar mass yields 0.05 moles, which multiplies by the standard molar volume of 22.4 dm³ to produce 1.12 dm³ of nitrogen gas. Common mistake: Misapplying the molar volume constant for gases at STP.

A student could guess simpler coefficients like 2 and 1 by balancing elements haphazardly without checking charges. Balancing the manganese dioxide and hydrochloric acid equation properly requires 4 moles of HCl and 2 moles of H2O, giving x = 4 and y = 2. Common mistake: Balancing only the metal atoms while neglecting hydrogen and oxygen stoichiometry.

Options specifying 1000 g of water are common distractors designed to test the difference between molality and molarity. A 1 M solution requires 40 grams of sodium hydroxide dissolved in enough water to make exactly 1000 cm³ of total solution volume. Common mistake: Confusing molarity (per liter of solution) with molality (per kilogram of solvent).

Carbon and zinc might seem unreactive if one forgets their specific reactivities with high-temperature vapor. Copper and bromine are completely unreactive toward water or steam under normal conditions, unlike zinc which reacts with steam and carbon which interacts at high heat. Common mistake: Overlooking the specific behavior of transition metals and halogens in the presence of water vapor.

Linear or exponential choices often appeal to students who confuse algebraic growth patterns with gas laws. Boyle’s law dictates that pressure and volume maintain an inverse relationship at constant temperature, which graphs geometrically as a hyperbola. Common mistake: Confusing an inverse proportionality curve with a direct linear relationship.

Decomposition or chemical oxidation might seem plausible if one mistakes phase changes for chemical reactions. Melting provides thermal energy that overcomes intermolecular forces, allowing the rigid molecules to become mobile in the liquid phase without altering their chemical identity. Common mistake: Assuming that a change of state involves breaking chemical bonds rather than intermolecular forces.

A student might incorrectly choose a 1:1 ratio by blindly comparing the mass numbers without converting to moles. Calculating the moles shows 1 mole of hydrogen gas and 0.5 moles of oxygen gas, which establishes a molecular ratio of 2:1. Common mistake: Comparing mass grams directly instead of converting to moles to find the true ratio of molecules.

A student might incorrectly choose option C or D because they recognize some valid rate-enhancing factors while missing others, such as overlooking the surface area effect of a powdered solid or the role of product removal. To determine the correct combination, evaluate each factor: a catalyst successfully reduces activation energy, a higher temperature raises collision frequency between particles, and a powdered solid enlarges the surface area for contact. Combining items 1, 3, and 5 provides the complete set of valid reaction rate boosters. Common mistake: Forgetting that a powdered solid increases surface area to speed up reactions.