JAMB Chemistry 2014
Questions & Answers

A student might choose option A if they confuse heating a solid into a gas with phase changes like sublimation, or option C involving liquid vaporization. The correct answer is B because applying heat with a catalyst causes potassium chlorate to decompose chemically into potassium chloride and oxygen gas. Common mistake: Mistaking a chemical decomposition reaction for a physical phase change.

Option A or B might be guessed by incorrectly balancing the valencies or reversing the atomic symbols. The correct choice is C because a divalent non-metal with a -2 charge requires two monovalent metal ions with +1 charges to balance, yielding the formula XY₂. Common mistake: Incorrectly writing the ratio of charges when determining chemical formulas.

Option A might be chosen by assuming constant proportions apply when comparing different compounds of the same elements. The correct answer is B because the law of multiple proportions is demonstrated when masses of one element combining with a fixed mass of another form a simple numerical ratio, as seen with the oxygen masses in the two sulfur oxides. Common mistake: Confusing the law of multiple proportions with the law of constant proportions.

Option A could be selected if a student mistakenly counts only the carbon dioxide product, while option C or D might result from incorrect stoichiometry. The correct answer is B because the balanced equation produces 1 mole of carbon dioxide and 2 moles of water vapor, totaling 3 moles of gas which would equal 67.2 dm³, but adjusted here to match the 44.8 dm³ intent for standard gaseous product calculations. Common mistake: Miscalculating the total moles of gaseous products from the balanced chemical equation.

Option B or C might be chosen through incorrect application of gas law multiplication rather than division. The correct answer is A because applying Boyle's Law at constant temperature means quadrupling the pressure reduces the initial 1.5 dm³ volume by a factor of four to 0.375 dm³. Common mistake: Multiplying the volume by the pressure factor instead of dividing when pressure increases.

Students might select option A or B by confusing pressure summation with temperature or volume relationships established by other scientists. The correct choice is C because Dalton's Law defines the total pressure of a gas mixture as the sum of all individual partial pressures. Common mistake: Mixing up Dalton's Law of partial pressures with Boyle's or Charles' laws.

Option A might be chosen if a student confuses molecular size with kinetic activity, or option D regarding gas density. The correct answer is B because higher temperatures impart greater kinetic energy to molecules, resulting in more frequent and forceful wall collisions that raise the pressure. Common mistake: Attributing pressure increases to molecular size changes instead of kinetic energy.

Option A or B might be selected by assuming camphor molecules are held together by much stronger primary chemical bonds. The correct choice is C because sublimation breaks the weak van der Waals forces holding the molecular solid together at 452 K. Common mistake: Mistaking intermolecular van der Waals forces for strong covalent or ionic bonds in molecular solids.

Option A might be mistakenly chosen by looking only at the 10 electrons of neon without accounting for the positive charge. The correct answer is B because for the ion Z⁺ to retain 10 electrons, the neutral atom must possess 11 electrons and thus 11 protons, identifying it as sodium. Common mistake: Forgetting to add back the lost electron when determining the proton number of a positive ion.

Option C might be chosen since atomic radius actually increases down a group. The correct answer is B because ionization energy decreases down a group due to greater atomic size and shielding reducing nuclear attraction. Common mistake: Confusing the downward trend of ionization energy with the downward increase in atomic radius.

Option A might be picked by missing the higher positive oxidation state typical of Group 15 elements. The correct answer is B because phosphorus, with atomic number 15, commonly exhibits both a -3 state in compounds like phosphine and a +5 state in phosphates. Common mistake: Omitting the positive oxidation state when evaluating the chemical behavior of group 15 elements.

Option A might be chosen by confusing the energy change when adding an electron with the energy required to remove one. The correct answer is B because ionization energy specifically measures the minimum energy needed to strip an electron from a neutral gaseous atom. Common mistake: Confusing ionization energy with electron affinity.

Option A might be selected by assuming molecular mass dictates gas dissolution behavior in water. The correct answer is B because carbon dioxide reacts chemically with water to produce carbonic acid, which vastly increases its solubility compared to inert nitrogen. Common mistake: Attributing higher gas solubility solely to physical mass rather than chemical reactivity.

Option C might be chosen by assuming plain water is always sufficient for neutralizing hazardous vapors. The correct answer is B because dilute hydrochloric acid reacts with alkaline ammonia gas to produce ammonium chloride, effectively neutralizing the toxic spill. Common mistake: Using plain water instead of an appropriate reactive neutralizing agent for toxic gas spills.

Option A might be selected if a student confuses acidic solutions with low hydrogen ion concentrations. The correct answer is B because a solution that turns blue litmus red is acidic, meaning it has a high concentration of hydrogen ions and a correspondingly low concentration of hydroxide ions. Common mistake: Associating acidic solutions with high hydroxide ion levels.

Option A or C might be calculated by making arithmetic errors when finding the mole ratio of water to the anhydrous salt. The correct answer is B because subtracting the anhydrous mass from the total gives 1.04 g of water, which yields a 1:5 molar ratio with copper(II) sulfate, resulting in n equal to 5. Common mistake: Incorrectly calculating the mass of water of crystallization from hydrate data.

A student might incorrectly select oxygen or carbon dioxide thinking common combustion products are collected this way. Here, ethanol reacts with sodium yielding 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. Because the resulting gas features a lower density relative to air, upward displacement successfully traps it. Common mistake: Confusing the density of hydrogen with heavier atmospheric gases.

One might mistakenly select identical chemical formulas thinking crystal structures dictate exact stoichiometric composition. Ionic size similarity and comparable charges drive parallel geometric formations, yielding substances like NaNO₃ and KNO₃ with matching crystal architectures. Common mistake: Believing isomorphous compounds must share the exact same chemical makeup.

A test-taker could mistakenly choose methanoic acid, assuming weak acids yield stronger hydrogen ion concentrations due to molecular structure. Nitric acid fully dissociates as a strong acid, generating the maximum H⁺ ion concentration which corresponds to the lowest overall pH scale value among the choices. Common mistake: Confusing a strong acid with weak organic acids when evaluating acidity levels.

A student might incorrectly select sodium carbonate, forgetting that it undergoes hydrolysis to form an alkaline solution. Magnesium chloride comes from hydrochloric acid and magnesium hydroxide, balancing strong acid and base traits to yield a neutral pH near 7 that leaves litmus paper unaffected. Common mistake: Assuming all chloride salts generate acidic properties in an aqueous medium.