JAMB Chemistry 2024 Past Questions & Answers

This question is based on Graham's Law of Diffusion, which states that the rate of diffusion of a gas is inversely proportional to the square root of its molar mass. Since gas X diffuses twice as fast as gas Y, the ratio of their rates is 2. To find the ratio of their masses, you must square this rate ratio (2² = 4), which shows that gas Y is four times as heavy as gas X.

Common mistake: Students often mistakenly choose option A because they forget the relationship involves the square root of the mass and directly link 'twice as fast' to 'two times as heavy'.

To solve this, first find the number of moles of helium by dividing the given mass by its molar mass (moles = mass/molar mass). The molar mass of helium is 4 g/mol, so you have 11.5g / 4 g/mol = 2.875 moles. Next, multiply the number of moles by Avogadro's constant (6.02 x 10²³) to find the number of molecules. This gives 2.875 × 6.02 x 10²³ which is approximately 1.73 x 10²⁴ molecules.

Common mistake: Students might incorrectly handle the scientific notation, calculating 17.3 x 10²³ and then mistakenly choosing 1.73 x 10²³ (Option A) instead of the correct 1.73 x 10²⁴.

Alnico is a special type of alloy known for being a very strong permanent magnet. The name 'Alnico' is a clue to its main components, which are Aluminium (Al), Nickel (Ni), and Cobalt (Co). Therefore, the missing constituent in the list is Cobalt (Co).

Common mistake: Students might mistakenly choose Copper (Cu) or Manganese (Mn) because they are also common metals used in other well-known alloys like brass and steel.

Noble metals are metals that are very resistant to corrosion and oxidation in moist air. Because Silver and Gold are very unreactive, they do not easily combine with other elements in nature. This property allows them to be found as free, uncombined elements in the earth's crust, earning them the name 'noble metals'.

Common mistake: Students might mistakenly choose 'natural metals' because these elements are found naturally, but 'noble metals' is the specific scientific term for unreactive metals like gold and silver.

The question asks about a specific step in the industrial extraction of a metal, Aluminium. This process, known as metallurgy, is a key part of the study of metals and their compounds. The removal of impurities like silica from the bauxite ore is a chemical purification step that must occur before the final extraction, and it involves specific chemical reactions of the metal's ore.

Common mistake: Students often mistakenly choose electrolysis because it is the famous final process for extracting pure aluminium metal, but they forget that the ore must be chemically purified first to remove impurities like silica.

Calcium ethynide, more commonly known as calcium carbide (CaC₂), undergoes hydrolysis when it reacts with water. The balanced chemical equation for this reaction is CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂. The gaseous product formed is C₂H₂, which is an alkyne called ethyne. This reaction is the standard laboratory method for preparing ethyne gas.

Common mistake: Students often confuse the reaction of calcium carbide with that of aluminium carbide (Al₄C₃), which reacts with water to produce methane (CH₄).

Petroleum (crude oil) is separated into different useful parts called fractions by fractional distillation. Bitumen, also known as asphalt, is the thick, black, and sticky residue left at the bottom of the fractionating column because it has the highest boiling point. Due to its sticky and waterproof nature, it is mixed with sand and gravel to surface roads.

Common mistake: Students might mistakenly choose petrol or diesel because they are very common petroleum fractions, but these are lighter liquids used as fuel for vehicles, not for road surfacing.

Fractional distillation separates crude oil into useful parts called fractions based on their different boiling points. Fractions with lower boiling points are collected at the top of the column, while those with higher boiling points are collected at the bottom. Kerosene, which is used for jet fuel and cooking, distils in the temperature range of about 175°C to 325°C, making it the correct fraction for the 200–250°C range.

Common mistake: Students often confuse the boiling ranges of kerosene and diesel, but diesel has a higher boiling point and is collected at a higher temperature range (250–350°C).

Fractional distillation of coal tar is a major industrial process used to obtain aromatic hydrocarbons. Benzene is the simplest aromatic hydrocarbon and is a primary liquid product obtained from this process. It serves as a crucial starting material, or feedstock, for synthesizing a wide range of organic compounds used in industries, including the pharmaceutical industry to make drugs.

Common mistake: Students may mistakenly choose Toluene because it is also an aromatic hydrocarbon obtained from coal tar, but benzene is the more fundamental compound and a more significant primary feedstock for the chemical industry.

First, find the empirical formula by dividing the percentage of each element by its atomic mass and then finding the simplest whole number ratio, which gives C₅H₇NO₂. Next, calculate the molar mass using the formula: Molar Mass = 2 x Vapour Density, which is 2 x 56.5 = 113 g/mol. The mass of the empirical formula C₅H₇NO₂ is (12x5)+(1x7)+14+(16x2) = 113 g/mol. Since the empirical formula mass equals the molar mass, the molecular formula is the same as the empirical formula, C₅H₇NO₂.

Common mistake: Students often forget that the molar mass is twice the vapour density $(2 x 56.5 = 113)$, and might incorrectly use the vapour density value directly as the molar mass, leading to a wrong calculation.

First, calculate the mass of the empirical formula by adding the mass contributions of X and Y, which is 72 + 6 = 78. This value is the simplest possible Relative Molecular Mass (RMM) for the compound. There is a direct relationship between RMM and Vapour Density (VD), given by the formula RMM = 2 x VD. To find the vapour density, we simply divide the RMM by 2, which gives 78 / 2 = 39.

Common mistake: Students often calculate the relative molecular mass (78) correctly but forget to divide by 2 to find the vapour density, leading them to incorrectly choose option D.

The Biuret test is a specific chemical test used to identify the presence of peptide bonds in a substance. Proteins are large molecules (macromolecules) made up of amino acids joined together by peptide bonds. When Biuret solution is added to a sample containing protein, the copper(II) ions in the reagent react with the peptide bonds to form a purple or violet-coloured complex, indicating a positive result.

Common mistake: Students often confuse the chemical tests for different biological macromolecules, and might mistakenly associate the Biuret test with carbohydrates instead of the correct test, which is usually Benedict's test for reducing sugars or the iodine test for starch.

The gas described is ethyne, commonly known as acetylene (C₂H₂). When ethyne is burned with a supply of pure oxygen, it produces an extremely hot flame called the oxy-acetylene flame, which can reach temperatures over 3000°C. This intense heat is sufficient to melt steel, making it the standard flame used for cutting and welding metals.

Common mistake: Students may mistakenly choose an option involving a miners' lamp because they recall that acetylene was historically used in them, but they overlook the 'very hot' description which points to the modern industrial application of welding.

Butene (C₄H₈) has structural isomers, but only but-2-ene can show geometrical isomerism. This is because for geometrical (cis-trans) isomerism to exist, each carbon atom in the double bond must be attached to two different groups. But-2-ene (CH₃-CH=CH-CH₃) meets this condition, giving two isomers: cis-but-2-ene and trans-but-2-ene. But-1-ene does not show geometrical isomerism, so the total number of geometrical isomers for butene is 2.

Common mistake: Students often mistakenly count all the structural isomers of C₄H₈ (but-1-ene, but-2-ene, and 2-methylpropene), arriving at 3, but the question specifically asks for geometrical isomers.

Isomers are compounds with the same molecular formula but different structural arrangements. The formula C₃H₈O can represent two different types of functional groups: alcohols and ethers. The possible isomers are propan-1-ol, propan-2-ol (which are alcohols), and methoxyethane (which is an ether), making a total of three isomers.

Common mistake: Students often only consider the positional isomers of the alcohol (propan-1-ol and propan-2-ol) and forget the functional group isomer, which is an ether (methoxyethane), leading them to incorrectly choose 2.

Millon's test is a specific chemical test used to identify the presence of proteins containing the amino acid tyrosine. Egg-white is rich in a protein called albumin, which contains tyrosine. When Millon's reagent is added to the egg-white solution and heated, it first forms a white precipitate which then turns brick red, indicating a positive result for protein.

Common mistake: Students may confuse the result with other food tests, such as the Biuret test which gives a violet colour, or they might select 'reddish brown' which is a close but incorrect description of the final colour.

First, we need to compare the concentration of the given solution with its solubility in the same units. The solubility is given as 3.10 mol/dm³. We must convert the solution's concentration from g/dm³ to mol/dm³ by dividing by the molar mass of KNO₃ (K=39, N=14, O=16), which is 101 g/mol. So, the concentration is 303 g/dm³ ÷ 101 g/mol = 3.00 mol/dm³. Since the concentration of the solution (3.00 mol/dm³) is less than the solubility limit (3.10 mol/dm³), it means more solute can still be dissolved, making the solution unsaturated.

Common mistake: Students may mistakenly choose 'saturated' by assuming the given concentration of 303 g/dm³ is equivalent to the solubility of 3.10 mol/dm³ without performing the necessary unit conversion.

To find the solubility in mol/dm³, you first need to calculate the number of moles of sodium chloride (NaCl). The molar mass of NaCl is the sum of the atomic masses of Na and Cl, which is 23 + 35.5 = 58.5 g/mol. Next, convert the given mass to moles using the formula: Moles = Mass / Molar Mass = 127g / 58.5 g/mol = 2.17 moles. Since this amount is dissolved in 1.0 dm³ of water, the solubility is 2.17 moles / 1.0 dm³ = 2.17 mol/dm³, which rounds to 2.2 mol/dm³.

Common mistake: The most common mistake is forgetting to convert the given mass of sodium chloride (127g) into moles before calculating the concentration, which is required to get the answer in mol/dm³.

First, find the number of moles of silver to be deposited. Moles = mass / molar mass = 180g / 108g/mol = 1.667 moles. The reaction for the deposition of silver is Ag⁺ + e⁻ → Ag, which means 1 mole of electrons (1 Faraday) is needed to deposit 1 mole of silver. Therefore, to deposit 1.667 moles of silver, you will need 1.667 Faradays (F) of electricity, which is approximately 1.67F.

Common mistake: Students might mistakenly assume the charge on the silver ion is 2+ (Ag²⁺), leading them to incorrectly multiply the number of moles by 2 to find the Faradays required.

In electroplating, the object to be coated, in this case the spoon, is always made the cathode (negative electrode). This is so that the positive metal ions from the electrolyte, which are silver ions (Ag+), are attracted to it and deposited as a thin layer of silver metal. The anode (positive electrode) must be made of the plating metal, pure silver, to dissolve and replace the silver ions being used up from the electrolyte solution.

Common mistake: Students often confuse the anode and cathode, incorrectly making the object to be plated (the spoon) the anode instead of the cathode where reduction and deposition occur.

To write the correct formula for an ionic compound, the total positive charge from the cations must balance the total negative charge from the anions. The table shows the Iron(III) ion is Fe³⁺ and the Bromide ion is Br⁻. To create a neutral compound, one Fe³⁺ ion (with a +3 charge) needs three Br⁻ ions (each with a -1 charge), making the correct formula FeBr₃. Therefore, the formula given in option C, Fe₃Br, is incorrect.

Common mistake: Students might get confused by the more complex formula in option A, Al₂(SO₄)₃, and incorrectly assume it is wrong due to the brackets and multiple subscripts.

Sulphuric acid (H₂SO₄) is a strong dibasic acid, meaning it completely ionizes in water to produce two hydrogen ions (H⁺) for every one molecule of acid. Therefore, the concentration of H⁺ ions is twice the concentration of the acid, which is 2 x 0.001 = 0.002 mol dm⁻³. The pH is calculated as the negative logarithm of the hydrogen ion concentration, so pH = -log[H⁺] = -log(0.002). This simplifies to 3 - log(2), and since log(2) is given as 0.3, the pH is 3 - 0.3 = 2.7.

Common mistake: The most common mistake is to calculate the pH directly from the acid's concentration (-log(0.001) = 3), forgetting that sulphuric acid is dibasic and provides two H⁺ ions per molecule.

Strong acids release many hydrogen ions in water, while weak acids release only a few. This difference in ion concentration can be measured by conductivity, pH, and the heat produced during a reaction. However, litmus paper only indicates if a substance is an acid by turning red; it cannot measure the strength of the acid. Since both strong and weak acids will turn blue litmus paper red, this method cannot be used to tell them apart.

Common mistake: Students may mistakenly believe that a stronger acid will cause a 'brighter' or 'deeper' red colour on litmus paper, but litmus is a simple qualitative indicator that does not show degrees of acidity.

To find the formula, we first determine the charge of the complex ion, hexacyanoferrate(II). The Roman numeral (II) tells us the iron (Fe) has a +2 charge, while the six ('hexa') cyanide (CN) ligands each have a -1 charge, giving a total of -6. The overall charge of the complex ion is (+2) + (-6) = -4, so the ion is [Fe(CN)₆]⁴⁻. To form a neutral compound with potassium (K⁺), we need four potassium ions to balance the -4 charge of the complex ion, which makes the correct formula K₄Fe(CN)₆.

Common mistake: Students often mistake the oxidation state of iron, using +3 instead of +2, which would incorrectly lead them to choose K₃Fe(CN)₆, the formula for potassiumhexacyanoferrate(III).

Gibbs Free Energy (ΔG) is a measure that determines if a chemical reaction can occur on its own without external energy input. For a reaction to be spontaneous (meaning it can happen by itself), the system must lose free energy. This loss of energy is represented by a negative value for the change in Gibbs Free Energy (ΔG).

Common mistake: Students often mistakenly choose 'zero' because they remember that ΔG is zero for a reaction at equilibrium, but the question asks about the condition for a reaction to be spontaneous, not for it to be at equilibrium.