JAMB Chemistry 2002 Past Questions & Answers

To solve this, we must find the total number of atoms in each option by multiplying the number of moles by the number of atoms in each molecule. A) Chlorine (Cl₂): 4 mol × 2 atoms = 8 moles of atoms. B) Ozone (O₃): 3 mol × 3 atoms = 9 moles of atoms. C) Butane (C₄H₁₀): 1 mol × $(4+10)$ atoms = 14 moles of atoms. D) Argon (Ar): 7 mol × 1 atom = 7 moles of atoms. The question asks for the least number of atoms, which is 7 moles (Argon). However, the provided correct answer is C, which has the most atoms. This indicates a common error in the question, where it likely intended to ask for the 'most' atoms, or confused 'atoms' with 'molecules' (where 1 mole of butane would be the least).

Common mistake: The most common mistake is to correctly answer the question as written by calculating that 7 moles of argon contains the least number of atoms (7 moles of atoms), which would be option D, but this is marked incorrect by the flawed answer key.

To find the chemical formula, first calculate the percentage of oxygen: 100% - 31.91% - 28.93% = 39.16%. Next, divide the percentage of each element by its atomic mass to find the mole ratio $(K: 31.91/39, Cl: 28.93/35.5, O: 39.16/16)$, which gives approximately 0.818 : 0.815 : 2.448. To get the simplest whole number ratio, divide all values by the smallest number (0.815), resulting in a ratio of 1:1:3, which gives the formula KClO₃.

Common mistake: Students often make calculation or rounding errors when converting the mole ratios to whole numbers, for instance, incorrectly rounding the oxygen ratio to 4 and choosing KClO₄.

The molecular formula for ethanoic acid is C2H4O2, which shows the actual number of atoms in one molecule. The empirical formula is the simplest whole-number ratio of atoms in a compound. To find this for ethanoic acid, you divide the subscripts in C2H4O2 by their greatest common divisor, which is 2, giving you CH2O.

Common mistake: Students may mistakenly choose 'Molecular formula' because they forget that the molecular formula shows the actual number of atoms (C2H4O2), while the empirical formula shows the simplest ratio.

When two miscible liquids are mixed, the boiling point of the mixture is typically between the boiling points of the two pure liquids. Since a small amount of trichloromethane (b.p. 60°C) is added to a large amount of ethanol (b.p. 78°C), the mixture's boiling point will be slightly lower than that of the main component, ethanol. Therefore, the most probable boiling point will be close to but less than 78°C, making the range 70°C – 74.4°C the correct answer.

Common mistake: Students may mistakenly choose option B, thinking that mixing two liquids will always result in a boiling point higher than the individual components, which is only true for specific mixtures called maximum boiling azeotropes.

Chromatography is a technique used to separate the different components of a mixture, like the different colours in an ink. The separation works because as a solvent moves through a medium (like paper), it carries the ink components along. However, some components are more attracted to the paper or dissolve better in the solvent, which makes them move at different speeds and separate into distinct bands.

Common mistake: Students might mistakenly choose option C, thinking about solubility, but the core principle of separation is the *different rates* of movement, not the components dissolving into one another.

Anhydrous cobalt(II) chloride is a chemical used to test for the presence of water, and it is blue in colour. When it absorbs water, for example from steam, it becomes hydrated cobalt(II) chloride. The hydrated form of this salt has a distinct pink colour, so the overall colour change is from blue to pink.

Common mistake: Students may confuse this reaction with the test for water using anhydrous copper(II) sulphate, which changes from white to blue.

Magnesium is a reactive metal that displaces hydrogen from acids. For this reaction to occur, the solution must be acidic, meaning it has a significant concentration of hydrogen ions (H+). Using the formula [H+][OH-] = 10⁻¹⁴, we can find the H+ concentration for each option. For option D, with [OH-] = 1.0 x 10⁻¹² mol dm⁻³, the hydrogen ion concentration [H+] is 10⁻¹⁴ / 10⁻¹² = 1.0 x 10⁻² mol dm⁻³, which is an acidic solution (pH 2) strong enough to react with magnesium and liberate hydrogen gas.

Common mistake: Students may mistakenly choose option A, thinking that the highest concentration value (1.0 x 10⁻²) will lead to a reaction, without realizing this is for OH⁻ ions, which makes the solution strongly basic and unreactive.

Silicate minerals are the main components of rocks, sand, and soil, which are naturally abundant and make up most of the Earth's crust. They are generally considered stable, inert, and non-toxic in their natural form. The other options are all well-known pollutants: uranium is radioactive, lead compounds are toxic heavy metals, and organophosphorous compounds are often used as harmful pesticides.

Common mistake: Students might be unfamiliar with the term 'silicate minerals' and instead choose a substance like Uranium, incorrectly assuming its pollution is less widespread than that from lead or pesticides.

The correct option is actually A. Alcohols contain a hydroxyl (-OH) group, which is polar. The hydrogen atom in this group can form a strong intermolecular attraction, known as a hydrogen bond, with the oxygen atoms of water molecules. This hydrogen bonding is the primary reason why lower alcohols are very soluble in water.

Common mistake: Students may incorrectly choose 'Ionic character' because the O-H bond is polar, but alcohols are covalent molecules, not ionic, and their solubility is due to hydrogen bonding.

The 'furring' of kettles is the formation of a hard, chalky deposit called limescale. This happens when water containing temporary hardness is boiled. Temporary hardness is caused by the presence of dissolved calcium hydrogentrioxocarbonate(IV), which is soluble in water. When heated, it decomposes to form insoluble calcium trioxocarbonate(IV), which is the 'fur' or scale that sticks to the kettle.

Common mistake: Students often mistakenly choose Calcium trioxocarbonate(IV) because it is the chemical name for the fur itself, but the question asks for the substance present in the water *before* boiling.

Concentrated tetraoxosulphate(VI) acid is a very strong dehydrating agent, meaning it aggressively removes water from other substances. When it touches the skin, which is composed of organic compounds and water, it rapidly pulls water molecules out of the skin cells. This removal of water, known as dehydration, destroys the tissue and causes a severe chemical burn.

Common mistake: Students may mistakenly choose 'Hydrolysis' because it also involves water, but hydrolysis is the chemical breakdown of a substance by water, not the removal of water from a substance.

Efflorescence is the correct term for the process where a hydrated salt loses its water of crystallization when exposed to the air. This occurs because the water vapour pressure of the salt is greater than that of the surrounding atmosphere, causing the water molecules to escape. A common example is washing soda (sodium carbonate decahydrate) which turns into a white powder upon exposure.

Common mistake: Students often confuse efflorescence with deliquescence, which is the opposite process where a substance absorbs moisture from the air to form a solution.

The initial solution has a pH of 8.4, which means it is already slightly basic since its pH is greater than 7. Sodium hydroxide (NaOH) is a strong base, so adding it to the solution increases the concentration of hydroxide ions (OH⁻). This makes the solution even more basic, which causes the pH value to increase. Therefore, the resulting pH will be greater than 8.4.

Common mistake: Students might mistakenly choose 'Less than 8.4' because they confuse the effect of adding a base with adding an acid, which is what would lower the pH.

First, calculate the concentration of the prepared solution. The moles of the salt is mass (3.40g) divided by molar mass (101g/mol), which is 0.0337 mol. The volume is 250cm³, which is 0.25dm³. Therefore, the concentration is 0.0337 mol / 0.25dm³ = 0.135 mol/dm³. Since this concentration is less than the given solubility (0.34 mol/dm³), the solution can still dissolve more salt and is therefore unsaturated.

Common mistake: Students may mistakenly choose 'Saturated' if they perform the calculation incorrectly, perhaps by forgetting to convert the volume from 250cm³ to 0.25dm³, leading to an incorrect concentration value.

First, write the balanced chemical equation for the reaction: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. This shows that the mole ratio of acid (na) to base (nb) is 2:1. Using the titration formula CaVa / CbVb = na / nb, we can find the concentration of the acid (Ca). Substituting the given values: (Ca × 20cm³) / (0.2 mol dm⁻³ × 25cm³) = 2 / 1, which simplifies to Ca = $(0.2 × 25 × 2)$ / 20, resulting in 0.5 mol dm⁻³.

Common mistake: Students often forget to use the correct 2:1 mole ratio from the balanced equation, incorrectly assuming a 1:1 reaction which leads to a wrong answer.

The degree of ionic character in a bond depends on the properties of the metal ion (cation). A small cation with a high positive charge has a strong polarizing power, meaning it pulls the electron cloud of the non-metal towards itself, creating more covalent character. Among the choices, Aluminium forms the Al³⁺ ion, which has the highest charge (+3) and a small size, giving it the greatest ability to create a covalent-like bond with chlorine. Therefore, AlCl₃ has the most covalent character and the least ionic character.

Common mistake: Students might mistakenly choose LiCl, thinking that because Lithium is a Group 1 metal it will be the most different, but they forget to consider that the high +3 charge on the Aluminium ion is the most important factor here.

To find the partial pressure of oxygen, you first need to calculate its mole fraction in the mixture. The total number of moles is 0.75 (cyclopropane) + 0.66 (oxygen) = 1.41 moles. The mole fraction of oxygen is its own moles divided by the total moles, which is 0.66 / 1.41 ≈ 0.468. According to Dalton's Law, the partial pressure of a gas is its mole fraction multiplied by the total pressure, so the partial pressure of oxygen is 0.468 × 0.7 atm = 0.3276 atm, which is approximately 0.33 atmosphere.

Common mistake: A common mistake is to incorrectly multiply the moles of oxygen (0.66) by the total pressure (0.7), which leads to the wrong answer of 0.462 atm, close to option B.

The question asks for processes that release carbon(IV) oxide (CO2) into the atmosphere. Respiration is the process where living organisms breathe out CO2. Decay is when dead organic matter is broken down by bacteria, releasing CO2. Combustion is the burning of fuels like wood or petrol, which also produces a large amount of CO2. Therefore, all three processes in option C return carbon(IV) oxide to the atmosphere.

Common mistake: Students often mistakenly choose option A because they forget that photosynthesis *removes* carbon(IV) oxide from the atmosphere, while the question asks for processes that *add* it.

The question tests your knowledge of the properties of subatomic particles. Protons and neutrons are found in the nucleus and have a relative mass of approximately 1, while electrons orbit the nucleus and are much lighter. Specifically, the mass of a proton is about 1836 to 1840 times the mass of an electron, making option B the correct statement.

Common mistake: Students may mistakenly choose option D, confusing the definition of an atomic mass unit $(1/12 the mass of a single carbon-12 atom)$ with one-twelfth the molar mass of carbon, which is 1g/mol.

This question uses Graham's Law of Diffusion, which states that the rate of diffusion of a gas is inversely proportional to the square root of its relative molecular mass. The formula is Rₓ/Rᵧ = $√(Mᵧ/Mₓ)$. Since gas X diffuses twice as fast as gas Y, Rₓ/Rᵧ = 2, and we are given Mₓ = 28. Substituting these values, we get 2 = $√(Mᵧ/28)$, and squaring both sides gives 4 = Mᵧ/28, so Mᵧ = 4 × 28 = 112.

Common mistake: Students often choose C by forgetting to square the rate ratio (2) in the formula, mistakenly calculating the answer as 2 x 28 = 56.

This question applies Charles's Law, which states that at constant pressure, the volume of a gas is directly proportional to its absolute temperature (V₁/T₁ = V₂/T₂). To solve this, you must first convert temperatures to Kelvin by adding 273. Based on the provided correct option, it is highly probable the question had a typo and intended the temperatures to be 27°C and 45°C. Converting these gives T₁ = 27 + 273 = 300 K and T₂ = 45 + 273 = 318 K. Using the formula V₂ = (V₁ × T₂) / T₁, we get V₂ = (92 cm³ × 318 K) / 300 K = 97.28 cm³, which is approximately 97.0 cm³.

Common mistake: The most common mistake is forgetting to convert temperatures from Celsius to Kelvin, which leads to the incorrect calculation $(92 × 180)$ / 30 = 552.0 cm³, corresponding to option D.

Dalton's postulate that atoms of different elements combine in simple whole-number ratios to form compounds remains a fundamental principle of chemistry, as seen in formulas like H₂O and CO₂. The other options are incorrect because modern science has shown that atoms are divisible (into protons, neutrons, and electrons), can be changed in nuclear reactions, and can have different masses (isotopes).

Common mistake: Students often mistakenly choose option B because the law of conservation of mass applies to chemical reactions, but they forget that nuclear reactions can in fact change atoms.

Ordinary glass, also known as soda-lime glass, is manufactured by heating a mixture of three main raw materials. These are sand (silica, SiO2), limestone (calcium carbonate, CaCO3), and soda ash (sodium carbonate, Na2CO3). The sodium carbonate acts as a flux, which lowers the melting point of the silica, making it easier and more economical to melt and shape into glass.

Common mistake: Students may mistakenly choose K2CO3 because potassium carbonate is also used in glass making, but it is for producing a different, more expensive type of glass called hard glass or potash glass, not ordinary glass.

The reaction of the rock sample with acid produces a gas. This gas turns acidified potassium dichromate(VI) solution from orange to green, which is a classic test for a reducing agent. The green colour is due to the formation of the chromium(III) ion (Cr³⁺). Among the options, only the sulfite ion (SO₃²⁻) reacts with acid to produce sulfur(IV) oxide (SO₂), a gas that is a reducing agent and causes this specific colour change.

Common mistake: Students may incorrectly choose SO₄²⁻ because it is also a common sulfur-containing ion, but sulfates do not react with dilute acids to produce a gas.

The ability of a metal to react with a dilute acid to release hydrogen gas depends on its position in the electrochemical or activity series. Only metals that are above hydrogen in the series can displace it from an acid. Sodium (Na) is a highly reactive metal placed high above hydrogen, so it readily reacts with HCl to produce hydrogen gas. The other metals like Copper (Cu), Silver (Ag), and Gold (Au) are all below hydrogen in the series and are not reactive enough to displace it.

Common mistake: Students might mistakenly choose Copper (Cu) because it is a common metal, but they forget that it is below hydrogen in the activity series and therefore cannot displace hydrogen from dilute acids.