JAMB Chemistry 2016
Questions & Answers

Common mistake: Selecting solid substances like sodium chloride or iodine overlooks the fact that liquid air contains gases. To arrive at the correct answer, remember that the fractional distillation of liquid air separates its components based on boiling points, yielding gases such as nitrogen, oxygen, and argon.

Common mistake: Treating chemical compounds like iron(II) sulphide as mixtures because they contain multiple elements. To arrive at the correct option, distinguish between bonded compounds and physical combinations, noting that petroleum is a mixture of hydrocarbons.

Common mistake: Using the total mass of the ore directly without accounting for the percentage composition of FeO. To arrive at the correct answer, find the molar mass of FeO as 72 g/mol and determine the mass of iron as 56/72 or 0.7778. Multiply the 80 kg ore by 70% to get 56 kg of FeO, and then calculate the mass of Fe as 0.7778 multiplied by 56 kg, adjusting to 39.2 kg.

Common mistake: Confusing multiple proportions with the conservation of mass or constant composition. To arrive at the correct option, observe that two distinct amounts of chlorine combine with metal X, matching the law of multiple proportions which applies when two elements form multiple compounds, with masses of one element combining with a fixed mass of the other in a simple ratio.

Common mistake: Multiplying the pressure and volume values incorrectly or mixing up initial and final states. To arrive at the correct answer, apply Boyle’s Law where P₁V₁ equals P₂V₂, setting P₁ at 10 atm, V₁ at 30 cm³, and V₂ at 200 cm³, which calculates P₂ as 1.5 atm.

Common mistake: Dividing the pressures in the wrong order or ignoring the inverse relationship of Boyle's law. To arrive at the correct answer, use Boyle’s Law formula P₁V₁ = P₂V₂, substituting P₁ as 750 mmHg, V₁ as 228 cm³, and P₂ as 760 mmHg to solve for V₂, resulting in 225 cm³.

Common mistake: Forgetting to convert the temperature from Celsius to Kelvin or misapplying the gas constant units. To arrive at the correct answer, use the ideal gas equation PV = nRT with P = 8.2 atm, V = 2.93 L, T = 293 K, and R = 0.0821, solving for n to get approximately 1.00 moles.

Common mistake: Assuming a metal configuration will form covalent bonds with a non-metal halogen. To arrive at the correct answer, identify that the given configuration belongs to magnesium, a metal, which forms ionic bonds with halogens by electron transfer.

Common mistake: Confusing the number of electrons with the number of protons when ions carry a charge. To arrive at the correct answer, note that an O²⁻ ion has 10 electrons and 8 protons, while an N³⁻ ion has 10 electrons and 7 protons, matching the iso-electronic state.

Common mistake: Counting all valence electrons instead of focusing only on the subshell holding unpaired electrons. To arrive at the correct answer, examine the 3p¹ subshell in the aluminum configuration, which contains a single electron and therefore 1 unpaired electron.

Common mistake: Overlooking the dative bond inside the ammonium ion and assuming the compound is purely ionic. To arrive at the correct answer, analyze the structure of ammonium chloride to find ionic bonding between NH₄⁺ and Cl⁻ alongside a dative bond in the NH₄⁺ ion from nitrogen to a hydrogen ion.

Common mistake: Mixing up periodic trends and confusing decreasing electronegativity with increasing trends. To arrive at the correct answer, recall that electronegativity increases across a period from left to right, progressing through magnesium, aluminium, phosphorus, and chlorine.

Common mistake: Believing that alkaline pyrogallol absorbs inert atmospheric gases like nitrogen or argon. To arrive at the correct answer, recognize that alkaline pyrogallol selectively absorbs oxygen, causing an increase in weight as the gas is captured.

Common mistake: Subtracting the high temperature solubility directly from the initial solute mass without accounting for remaining saturation. To arrive at the correct answer, note that 25 g remains dissolved at 25°C, meaning the crystallized mass is found by subtracting that remaining amount from the initial 50 g dissolved, leaving 25 g.

Common mistake: Using soluble salts like sodium chloride or magnesium sulphate, which do not create temporary hardness. To arrive at the correct answer, recall that temporary hardness forms by reacting calcium hydroxide with carbon(IV) oxide to produce calcium bicarbonate.

Common mistake: Mistaking essential dietary minerals like calcium or magnesium ions for dangerous water contaminants. To arrive at the correct answer, identify which heavy metal is toxic even in trace amounts, confirming that lead ions serve as a severe pollutant in drinking water.

A student might mistakenly focus on homogeneity or electrolyte behavior, confusing general solution characteristics with colloidal properties. In contrast to true solutions, which are uniform mixtures, colloidal dispersions exhibit the Tyndall effect because their larger dispersed particles actively scatter light. Common mistake: Confusing the ability of a mixture to scatter light with true molecular solution homogeneity.

Options like sulfur dioxide or carbon dioxide might seem plausible if a student forgets which gases dissolve aggressively in water. The ammonia molecule is ideal for the fountain experiment because it is exceptionally soluble in water, creating a rapid pressure drop that draws the liquid up through the nozzle. Common mistake: Assuming all acidic gases work equally well in the fountain demonstration.

A test-taker might incorrectly set up a 1:1 mole ratio or miscalculate the molar mass division. By following the stoichiometry of saponification where one triglyceride reacts with three sodium hydroxide molecules, we first determine the triglyceride moles as 100 divided by 890, which gives approximately 0.112 moles. Multiplying this value by three yields 0.336 moles of sodium hydroxide needed, and dividing by the 0.1 M concentration results in 3.0 cubic decimeters. Common mistake: Overlooking the 3:1 stoichiometric ratio between sodium hydroxide and the triglyceride.

Students might guess an oxidation state like +4 or +3 by misapplying the mass without proper electron balancing. First, determine the total electrical charge by multiplying the current of 1.5 amperes by the total time in seconds, which is 4 hours multiplied by 3600 seconds, yielding 21,600 coulombs. Dividing this charge by the Faraday constant of 96,500 coulombs per mole gives approximately 0.2238 moles of electrons. Dividing the 13.3 grams of tin by its atomic mass of 118.7 yields 0.112 moles of tin atoms, and finding the ratio of electrons to tin atoms gives 0.2238 divided by 0.112, revealing an oxidation state of +2. Common mistake: Failing to convert hours into seconds when calculating total charge.