JAMB Chemistry 2017 Past Questions & Answers

Alkanones, also known as ketones, are a class of organic compounds that contain a carbonyl functional group (C=O) bonded to two carbon atoms. Their general formula is RCOR', where R and R' are alkyl groups. Option B correctly represents this structure, with the carbonyl group situated between two alkyl groups.

Common mistake: Students often confuse the formula for alkanones with that of alkanals (RCHO) because both contain the carbonyl functional group (C=O).

This question tests your knowledge of alloys, which are mixtures containing metals. Duralumin is an alloy composed mainly of aluminium (Al), copper, magnesium, and manganese. Alnico is an alloy made from aluminium (Al), nickel (Ni), and cobalt (Co). By comparing the elements in both alloys, it is clear that aluminium (Al) is the only element found in both.

Common mistake: Students might mistakenly choose Cobalt (Co) because they remember it is a key component of Alnico, without checking if it is also present in duralumin.

In modern atomic theory, an orbital is a region in an atom where there is a high probability of finding an electron. These orbitals have specific shapes and are named s, p, d, and f. The s-orbital is the simplest and has a non-directional, spherical shape, meaning it looks like a ball with the nucleus at its center.

Common mistake: Students sometimes confuse the three-dimensional spherical shape with its two-dimensional representation, a circle, leading them to incorrectly choose 'circular'.

A mordant is a substance used to fix a dye to a fabric, making the colour permanent and resistant to washing. Aluminium hydroxide is a gelatinous, sticky substance that binds with the fibres of the material and also with the dye molecules. This action 'fixes' the colour onto the fabric, which is why it is used as a mordant in the dyeing industry.

Common mistake: Students might mistakenly choose 'dye' because the question mentions the dyeing industry, but a mordant is a substance that helps the dye stick to the fabric, not the dye itself.

A tincture is a type of solution where the main solvent is alcohol. Tincture of iodine is a common antiseptic, which is prepared by dissolving iodine crystals in an alcohol. Among the given options, ethanol is the alcohol used for this purpose, often with some water as well.

Common mistake: Students often mistakenly choose water because it is a common solvent, but they forget that the specific term 'tincture' refers to a solution made with alcohol, not water.

Temporary hardness of water is caused by dissolved calcium hydrogen carbonate. This is formed when rainwater containing dissolved carbon (IV) oxide (which forms weak carbonic acid) flows over limestone or chalk deposits, which are mainly calcium carbonate (CaCO₃). The acid reacts with the insoluble calcium carbonate to form soluble calcium hydrogen carbonate, Ca(HCO₃)₂, which causes the temporary hardness.

Common mistake: Students often mistakenly choose CaSO₄ because it also causes water hardness, but it is responsible for permanent hardness, not the temporary hardness mentioned in the question.

An acid anhydride is a non-metal oxide that forms an acid when it reacts with water. When carbon (IV) oxide (CO₂) dissolves in water, it forms carbonic acid (H₂CO₃), which is a very weak acid that only slightly dissociates in water. The other options form stronger acids; for example, SO₃ forms the very strong sulphuric acid (H₂SO₄), and SO₂ forms sulphurous acid (H₂SO₃), which is a stronger weak acid than carbonic acid.

Common mistake: Students may mistakenly choose SO₂ because it also forms a weak acid (sulphurous acid), but they forget that carbonic acid (from CO₂) is a much weaker acid and the classic example taught.

Resonance is a concept used to describe the bonding in certain molecules where a single Lewis structure cannot fully represent the arrangement of electrons. It involves two or more equivalent structures, called resonance forms, that are in a conceptual equilibrium. The actual structure of the molecule is a hybrid or average of these forms, which leads to greater stability.

Common mistake: Students often mistakenly choose Isomerism because it also involves compounds with the same formula, but isomers are distinct, separate molecules, whereas resonance forms are different representations of a single molecule.

The preparation of ethyl ethanoate from ethanol and ethanoic acid is a reversible reaction called esterification. Concentrated sulphuric acid (H₂SO₄) is used as a catalyst to speed up the reaction. Importantly, it is also a powerful dehydrating agent, which means it absorbs the water formed as a product. By removing water, the equilibrium shifts to the right, increasing the amount of ethyl ethanoate produced.

Common mistake: Students often mistakenly choose Anhydrous CaCl₂ because it is a known drying agent, but it is unsuitable here as it reacts with the ethanol reactant.

Rusting is the specific term for the corrosion of iron, which is a slow oxidation process. For rusting to occur, two substances are essential: oxygen, which acts as the oxidizing agent, and water, which acts as the electrolyte or medium for the reaction. The iron is oxidized by oxygen in the presence of water to form hydrated iron(III) oxide, the reddish-brown substance we see as rust.

Common mistake: Students may mistakenly choose option D, thinking carbon dioxide (CO₂) is required, but while dissolved CO₂ can make water acidic and accelerate rusting, it is not a necessary reactant for the fundamental process itself.

The equilibrium constant expression, Kc, is defined as the ratio of the concentration of products to the concentration of reactants, with each concentration raised to the power of its stoichiometric coefficient from the balanced equation. For the reaction xP + yQ → mR + nS, the products are R and S, and the reactants are P and Q. Therefore, the correct expression is the concentration of products ([R]ᵐ[S]ⁿ) divided by the concentration of reactants ([P]ˣ[Q]ʸ).

Common mistake: Students often mistakenly choose option B, which incorrectly places the concentration of reactants in the numerator and products in the denominator.

This question is an application of Charles's Law, which states that the volume of a gas is directly proportional to its absolute temperature when pressure is constant. The formula is V₁/T₁ = V₂/T₂. Given V₁ = 2 dm³, T₁ = 300 K, and V₂ = 4 dm³ (doubled volume), we can find the new temperature T₂. Rearranging the formula gives T₂ = (V₂ × T₁) / V₁, so T₂ = (4 dm³ × 300 K) / 2 dm³ = 600 K.

Common mistake: Students sometimes forget the direct relationship and may incorrectly assume an additive process or misapply the formula, leading to a wrong answer like 400 K.

To find the oxidation number of iodine in KIO₃, you must know that the sum of oxidation numbers in a neutral compound is zero. The oxidation number of potassium (K) is +1 and oxygen (O) is -2. Letting the oxidation number of iodine (I) be 'x', the equation is (+1) + (x) + 3(-2) = 0. This simplifies to 1 + x - 6 = 0, which gives x = +5.

Common mistake: Students may forget to include the +1 oxidation state of potassium and incorrectly calculate the oxidation state for an 'IO₃' molecule, leading to an answer of +6.

Isomers are compounds that have the same molecular formula but different structural arrangements. The given molecular formula is C₅H₁₂, which is for pentane. We need to find an option that also has 5 carbon atoms and 12 hydrogen atoms but a different structure. 2-methyl butane has a main chain of 4 carbons (butane) and a methyl group (1 carbon) attached, making a total of 5 carbons, and its formula is C₅H₁₂, making it an isomer of C₅H₁₂.

Common mistake: Students often choose 2-methyl propane (D) by miscounting the total number of carbon atoms, not realizing it has only four carbons (C₄H₁₀) instead of the required five.

The chemical test described, using concentrated trioxonitrate (V) acid and warming to get a yellow colour, is called the Xanthoproteic test. This test is specifically used to detect the presence of proteins. The acid reacts with certain amino acids that make up the protein, causing the yellow colour. Option A shows the general structure of an amino acid, which is the basic unit of proteins.

Common mistake: Students might mistakenly choose option D (amide group) because the peptide bond that links amino acids in proteins is an amide linkage, but the Xanthoproteic test specifically identifies the aromatic side chains of certain amino acids, not the peptide bond itself.

First, we use the relationship between pH and pOH, which states that at 25°C, pH + pOH = 14. Given the pH is 3.80, we can find the pOH by calculating 14 - 3.80, which gives us 10.20. The pOH is the negative logarithm of the hydroxide ion concentration ([OH⁻]), so pOH = -log[OH⁻]. Therefore, the concentration of the hydroxide ion, [OH⁻], is equal to 10⁻ᵖᴼᴴ, which is 10⁻¹⁰.²⁰, and this calculates to approximately 6.3 × 10⁻¹¹ mol/dm³.

Common mistake: A common mistake is to calculate the hydrogen ion concentration $([H⁺] = 10⁻³.⁸⁰ ≈ 1.6 × 10⁻⁴)$ instead of the hydroxide ion concentration ([OH⁻]) that the question asks for.

The oxidation of primary alcohols like ethanol can produce two different products depending on the conditions. Incomplete oxidation, which uses a mild oxidizing agent, converts the ethanol into an aldehyde, which in this case is ethanal (CH₃CHO). If the oxidation were complete, using a strong oxidizing agent, the product would be a carboxylic acid, which is ethanoic acid (CH₃COOH).

Common mistake: Students often mistakenly choose option A, ethanoic acid, because they forget that it is the product of the *complete* oxidation of ethanol, not the incomplete oxidation.

A salt is formed from an acid and a base. When a salt from a weak acid and a strong base dissolves in water, the part from the weak acid reacts with water in a process called hydrolysis. This reaction produces hydroxide ions (OH⁻), and since the part from the strong base does not react, there is an excess of OH⁻ ions, making the solution alkaline.

Common mistake: Students often mistakenly choose 'an acidic solution' by confusing the rules, thinking the weak acid parent makes the salt solution acidic, when the opposite is true.

Ideal gas laws assume that gas particles have no volume and no forces of attraction between them. Real gases behave most like ideal gases at low pressure, because the particles are far apart, making their individual volume and attractive forces negligible. At high temperature, the particles have high kinetic energy, which allows them to easily overcome any weak attractive forces that might exist.

Common mistake: Students often confuse the conditions and may select high pressure, forgetting that this forces particles closer together and increases intermolecular forces, causing the gas to deviate from ideal behaviour.

The symbol ΔH stands for the change in enthalpy, which is the heat change in a reaction at constant pressure. A negative sign for ΔH means that the products have less energy than the reactants, indicating that heat has been released into the surroundings. By definition, a chemical reaction that releases heat is called an exothermic reaction.

Common mistake: Students often confuse the sign convention and choose 'endothermic', but endothermic reactions absorb heat from the surroundings and have a positive ΔH.

This question describes a nuclear reaction, specifically the alpha decay of Radium (Ra). An alpha particle is a helium nucleus, which has a mass number of 4 and an atomic number of 2 (represented as ⁴₂He). For the nuclear equation to be balanced, the sum of the mass numbers on the left side must equal the sum on the right side. Therefore, 226 = x + 4, and solving for x gives x = 226 - 4, which is 222.

Common mistake: A common mistake is choosing 226, which happens when a student forgets that emitting an alpha particle reduces the original mass number by 4.

In an ammonia (NH₃) molecule, the central nitrogen atom has five valence electrons and forms three single bonds with three hydrogen atoms, leaving one lone pair of electrons. According to VSEPR theory, the four electron pairs (three bonding pairs and one lone pair) arrange themselves in a tetrahedral geometry to minimize repulsion. However, the shape of the molecule is determined only by the positions of the atoms, which results in a trigonal pyramidal structure.

Common mistake: Students often mistakenly choose 'tetrahedral' because they confuse the electron pair geometry with the molecular shape, forgetting that the lone pair influences the shape but is not included when naming it.

This question is a direct application of Faraday's first law of electrolysis. First, calculate the total quantity of electricity (charge) passed using the formula Q = I × t, which is 10 A × 4830 s = 48300 C. From the reaction Ag⁺ + e⁻ → Ag, we know that 1 mole of electrons (which is 1 Faraday or 96500 C) is needed to deposit 1 mole of silver (108 g). By proportion, if 96500 C deposits 108 g of silver, then 48300 C will deposit $(48300 / 96500)$ × 108 g, which calculates to 0.5 × 108 g = 54.0 g.

Common mistake: Students may mistakenly use a valency of 2 for silver, similar to copper, which would lead them to incorrectly calculate half the mass (27.0 g).

Tin is a metal that is relatively unreactive because it has a weak electropositive character. This means it does not easily give up its electrons to form positive ions, which is what happens when a metal reacts with oxygen in the air. Because it holds onto its electrons more strongly than highly reactive metals like sodium or potassium, it remains stable and unaffected at room temperature.

Common mistake: Students may confuse physical properties with chemical reactivity and incorrectly choose options like 'Low melting point' or 'High boiling point', which do not determine an element's tendency to react.

According to Faraday's first law of electrolysis, the quantity of electricity (Q) is equal to the amount in moles (n) multiplied by the charge on the ion (z) and the Faraday constant (F). For a silver salt, the silver ion is Ag⁺, so the charge (z) is 1. To find the moles, we rearrange the formula to n = Q / (z × F), which gives n = 9650 C / $(1 × 96500 C mol⁻¹)$. This calculation simplifies to $1/10$, which equals 0.1 moles.

Common mistake: Students often make a calculation error by incorrectly dividing the Faraday constant by the quantity of electricity $(96500 / 9650)$, which gives 10 (Option C).