JAMB Chemistry 2020
Questions & Answers

1) Distractor check: A student might mistakenly select filtered sea water, soft drink, or writing ink, assuming they contain impurities or varied components. However, these options are incorrect because their components are evenly distributed throughout, creating a uniform composition. 2) Reasoning to the answer: A homogeneous mixture is defined by having a uniform composition throughout. Soft drinks, writing ink, and filtered sea water all fit this description as their substances are thoroughly blended and evenly distributed. Conversely, flood water contains suspended particles such as mud, which results in a non-uniform appearance and classifies it as a heterogeneous mixture. 3) Common mistake: Assuming all liquids or solutions that are not clear water must be heterogeneous mixtures.

1) Distractor check: Students may choose options like 'metals have a high melting point' or 'the crystal lattice of metals is easily broken', assuming these directly explain the wide gap between phase changes. However, these choices fail to account for the specific behavior of chemical bonds during these states. 2) Reasoning to the answer: Vaporizing a metal requires supplying enough energy to completely overcome and break the metallic bonds, whereas melting only disrupts the crystal lattice while allowing the delocalized electrons to persist in the liquid state. Because the total destruction of metallic bonds demands vastly more energy than merely disrupting the lattice, the substance requires a much higher temperature to boil, resulting in a large temperature interval. 3) Common mistake: Confusing the energy required to disrupt a crystal lattice with the higher energy needed to break metallic bonds entirely.

1) Distractor check: A student might mistakenly select 0.5 mole by simply looking at the molarity of the solution, or 2.0 moles by miscalculating the stoichiometry without considering the given volume. 2) Reasoning to the answer: Since sulfuric acid is a diprotic acid, each single molecule releases two hydrogen ions upon dissociation. Multiplying the solution concentration of 0.5 mol/dm³ by the total volume of 1 dm³ yields 0.5 moles of the acid, and because each mole of H₂SO₄ yields two moles of H⁺, multiplying 0.5 by 2 gives a total of 1.0 mole of H⁺ ions. 3) Common mistake: Forgetting to account for the diprotic nature of H₂SO₄ when calculating the total moles of hydrogen ions.

1) Distractor check: A test-taker could mistakenly choose 2 moles or 8 moles by misinterpreting the stoichiometric coefficients in the balanced chemical equation. 2) Reasoning to the answer: The balanced chemical equation establishes a ratio where 2 moles of H₂S require 3 moles of O₂ for complete reaction. Scaling this relationship for 4 moles of H₂S involves multiplying 4 by the stoichiometric factor of 3/2, which yields a requirement of 6 moles of O₂. 3) Common mistake: Mixing up the ratio of reactants by placing the coefficients upside down in the stoichiometric calculation.

1) Distractor check: Students might pick 400 K by assuming a linear, non-proportional temperature increase, or miscalculate to get another incorrect value. 2) Reasoning to the answer: Charles’s Law dictates that volume is directly proportional to absolute temperature at a constant pressure, expressed as V₁/T₁ = V₂/T₂. Given an initial volume of 2 dm³ at 300 K, a doubling of the volume to 4 dm³ means the temperature must also double proportionally, resulting in T₂ equal to (4/2) multiplied by 300, which evaluates to 600 K. 3) Common mistake: Using Celsius temperatures instead of Kelvin when applying gas laws, or failing to scale the temperature directly with the volume change.

1) Distractor check: A student might choose 32.0 s by incorrectly using the molar mass directly as the time, or 10 s through poor arithmetic manipulation of the square root. 2) Reasoning to the answer: Graham’s Law relates the diffusion times and molar masses of two gases via the equation t₂/t₁ = √(M₂/M₁). Substituting the given values where t₁ is 50 seconds for O₂ with a molar mass of 32, and H₂ has a molar mass of 2, the ratio becomes the square root of 2 over 32, which simplifies to the square root of 1/16, or 1/4. Multiplying 50 seconds by this fraction yields 12.5 seconds for hydrogen to diffuse. 3) Common mistake: Inverting the molar masses inside the square root term of Graham's Law.

1) Distractor check: Students might select volume, pressure, or mass, thinking these macroscopic properties directly reflect molecular motion. However, these variables depend on container size or quantity rather than molecular energy. 2) Reasoning to the answer: According to the kinetic theory of gases, the absolute temperature of a substance directly dictates the average kinetic energy of its molecules, expressed by the relationship KE = (3/2)kT. Therefore, temperature serves as the direct measure of how fast the molecules are moving on average. 3) Common mistake: Confusing pressure, which measures force per unit area from collisions, with the actual average kinetic energy of the particles.

1) Distractor check: A student might mistakenly select an increase in the number of molecules or density, assuming the gas somehow gains matter when heated. 2) Reasoning to the answer: When a gas in a fixed volume is heated, its molecules gain kinetic energy and travel at higher velocities. These faster-moving particles strike the container walls more frequently and with greater force, which directly results in a higher gas pressure. 3) Common mistake: Believing that heating a gas changes its total mass or molecular count inside a closed container.

1) Distractor check: A student could easily select 92 protons and 238 neutrons by confusing the mass number with the neutron count, or invert the numbers to choose 146 protons and 92 neutrons. 2) Reasoning to the answer: The symbol ²³⁸₉₂U designates an atomic number of 92, which specifies the exact number of protons in the nucleus. The mass number is given as 238, representing the total sum of protons and neutrons, meaning that subtracting the 92 protons from the 238 total yields 146 neutrons. 3) Common mistake: Treating the superscript mass number as the number of neutrons instead of the total sum of protons and neutrons.

1) Distractor check: A student might choose 1 or 3 by miscounting the valence electrons or assuming every non-bonding pair of electrons counts differently. 2) Reasoning to the answer: An oxygen atom possesses 6 valence electrons in its outer shell. It forms two covalent bonds with hydrogen atoms by sharing 2 of these electrons, which leaves the remaining 4 unshared valence electrons arranged as 2 distinct lone pairs on the central oxygen atom. 3) Common mistake: Confusing the total number of non-bonding valence electrons (4) with the number of lone pairs (2).

1) Distractor check: Students might guess a neutron or deuterium by miscalculating the conservation of mass and atomic numbers across the nuclear equation. 2) Reasoning to the answer: Balancing the nuclear equation requires that the sum of the mass numbers and atomic numbers on both sides must be equal. For the mass numbers, 7 plus 1 equals 4 plus the mass of X, giving X a mass number of 4. For the atomic numbers, 3 plus 1 equals 2 plus the atomic number of X, giving X an atomic number of 2, which identifies the particle as ⁴₂He. 3) Common mistake: Arithmetic errors when summing the superscript mass numbers or subscript atomic numbers on either side of the arrow.

1) Distractor check: A student might select element S because it has many valence electrons and is located near the halogens, or choose P or Q by guessing randomly among the options. 2) Reasoning to the answer: Elements possessing 1 to 3 electrons in their outermost shells typically exhibit metallic properties because they readily lose these valence electrons to form positive ions. Since element R has 3 valence electrons, it falls into this category and is most likely a metal, akin to aluminum in Group 13. 3) Common mistake: Assuming elements with higher valence electron counts are more likely to be metals.

1) Distractor check: A test-taker might select options containing uncommon laboratory gases like NH₃ or HCl, or pick dust and chlorine, which are less representative of heavy industrial combustion and sulfur processing. 2) Reasoning to the answer: Industrial facilities frequently release hydrogen sulfide from sulfur-containing compounds, sulfur dioxide from burning sulfur-heavy fuels, and carbon oxides from combustion processes. These specific byproducts combine to make H₂S, SO₂, and oxides of carbon the most probable pollutants found in an industrial setting. 3) Common mistake: Associating all air pollution exclusively with vehicle exhaust rather than specific industrial processes.

1) Distractor check: A student might choose carbon(II) oxide or oxygen, confusing general atmospheric gases with those responsible for acid deposition. 2) Reasoning to the answer: Sulphur(IV) oxide gas reacts with water vapor present in the atmosphere to generate sulfurous acid, which can further oxidize into sulfuric acid and precipitate as acid rain. 3) Common mistake: Confusing carbon monoxide with sulfur dioxide as the primary precursor for acid rain formation.

1) Distractor check: A student might select temporary or permanent hardness, confusing disinfection processes with mineral removal methods like softening. 2) Reasoning to the answer: Town water supplies undergo chlorination primarily to act as a disinfectant that destroys harmful microorganisms and pathogens. Therefore, chlorine is added to render the water free from bacteria, ensuring it is safe for the public to drink. 3) Common mistake: Confusing water purification methods designed to kill pathogens with treatments used to remove dissolved mineral ions.

1) Distractor check: A student might select a subset of the roman numerals, such as missing pressure or temperature, forgetting that physical conditions strongly influence gas solubility. 2) Reasoning to the answer: The solubility of a gas is governed by multiple factors: the chemical nature of both the solute and solvent dictate compatibility, temperature changes alter gas solubility inversely, and pressure dictates solubility directly in accordance with Henry’s Law. Because all four listed factors apply, the correct selection includes I, II, III, and IV. 3) Common mistake: Overlooking pressure as a critical factor in the solubility of gaseous solutes compared to solids.

1) Distractor check: Students could mistakenly pick solid particles dispersed in liquid, confusing a true solution with a suspension or colloid. 2) Reasoning to the answer: By definition, an aqueous solution is formed when a solute—which may exist as a solid, liquid, or gas—becomes uniformly dissolved within a water solvent. 3) Common mistake: Confusing a homogeneous aqueous solution with heterogeneous dispersions like suspensions or colloids.

1) Distractor check: A student might mistakenly select 1.6 × 10⁻⁴ M by stopping the calculation after finding the hydrogen ion concentration, or pick another incorrect exponent. 2) Reasoning to the answer: Given a pH of 3.80, the hydrogen ion concentration [H⁺] is calculated as 10⁻³.⁸⁰, which equals approximately 1.58 × 10⁻⁴ M. Using the ion product constant of water (Kw = 1.0 × 10⁻¹⁴), the hydroxide ion concentration [OH⁻] is found by dividing Kw by [H⁺], resulting in 1.0 × 10⁻¹⁴ divided by 1.58 × 10⁻⁴, which gives approximately 6.3 × 10⁻¹¹ M. 3) Common mistake: Forgetting to divide the ion-product constant by the hydrogen ion concentration to find the hydroxide ion concentration.

1) Distractor check: A student might pick an option that places methanol ahead of toluene, or misjudge the acid strength of acetic acid relative to a strong mineral acid. 2) Reasoning to the answer: Toluene has no acidic protons and is essentially neutral. Methanol acts as a very weak acid with a high pKa of about 15, while acetic acid is a moderately weak organic acid with a pKa around 4.76. Hydrochloric acid is a fully dissociated strong mineral acid with a negative pKa, making the correct ascending order of acidity: toluene, methanol, acetic acid, and hydrochloric acid. 3) Common mistake: Assuming organic compounds like methanol are stronger acids than mineral acids due to the presence of an oxygen-hydrogen bond.

1) Distractor check: A student might choose K₂SO₄·Al₂(SO₄)₃·24H₂O by confusing complex salt hydration with acidity, or pick a basic salt like Zn(OH)Cl. 2) Reasoning to the answer: An acid salt is characterized by retaining one or more replaceable hydrogen ions within its formula. Sodium hydrogen sulfate contains a replaceable hydrogen atom, whereas the other choices are either double salts or basic salts containing hydroxyl groups. 3) Common mistake: Failing to distinguish between basic salts containing hydroxide groups and acid salts containing replaceable hydrogen atoms.