JAMB Chemistry 2003 Past Questions & Answers

First, write the balanced chemical equation for the reaction: 2KClO3(s) → 2KCl(s) + 3O2(g). From the equation, the mole ratio shows that 2 moles of KClO3 decompose to produce 3 moles of oxygen gas. Since the question starts with 2 moles of KClO3, exactly 3 moles of O2 will be formed. At s.t.p., 1 mole of any gas occupies 22.4 dm³, so the volume of 3 moles of O2 is 3 x 22.4 dm³ = 67.2 dm³.

Common mistake: Students often mistakenly assume a 1:1 mole ratio, leading them to calculate the volume for 2 moles of oxygen (44.8 dm³) instead of the correct 3 moles from the balanced equation.

A physical change is a change that affects the form of a substance, but not its chemical composition, and is usually reversible. Freezing ice-cream is a physical change because it only changes its state from liquid to solid; no new substance is formed and it can be reversed by melting. The other options are chemical changes because new substances are formed: burning kerosene produces carbon dioxide and water, and calcium reacts with water to form calcium hydroxide and hydrogen gas.

Common mistake: Students might mistakenly choose 'Dissolving calcium in water' because they associate the word 'dissolving' with a physical change, but in this case, the calcium chemically reacts with water to form new substances.

To find the percentage by mass of oxygen, first calculate the total molar mass of the compound Al2(SO4)3.2H2O. The molar mass is $(2*27)$ + $3*(32 + 4*16)$ + $2*(2*1 + 16)$ = 54 + 288 + 36 = 378 g/mol. Next, find the total mass of oxygen in the formula, which is $(3*4*16)$ from the sulphate part and $(2*16)$ from the water part, giving 192 + 32 = 224 g. Finally, divide the mass of oxygen by the total molar mass and multiply by 100: $(224 / 378)$ * 100 = 59.25%.

Common mistake: Students often make the mistake of only calculating the mass of oxygen in the aluminium sulphate part, Al2(SO4)3, and forgetting to include the oxygen from the water of crystallization (2H2O), which leads to the incorrect answer C.

Adsorption is the process where particles of a substance, in this case nicotine, stick to the surface of another material, which is the cigarette filter. The filter is designed with a large surface area to effectively trap these particles as the smoke passes through. This is a physical separation technique used to purify the smoke by reducing its nicotine content.

Common mistake: Students often confuse adsorption with absorption (Option D), but absorption involves a substance soaking into the entire volume of a material, whereas adsorption only happens on the surface.

To balance a chemical equation, the number of atoms of each element must be the same on both the reactant (left) and product (right) sides. On the right side, there are 4 H₂O molecules, which gives a total of 4 x 2 = 8 hydrogen atoms. Therefore, 'p' on the left side must be 8 to balance the hydrogen. With p=8, we now have 8 nitrogen atoms on the left. On the right side, there are 3 x 2 = 6 nitrogen atoms in 3Cu(NO₃)₂ plus 'x' nitrogen atoms in xNO, so we have the equation 8 = 6 + x, which means x = 2.

Common mistake: Students often choose option C because they incorrectly balance the nitrogen atoms by only looking at the 6 nitrogen atoms in 3Cu(NO₃)₂, forgetting to also account for the nitrogen atoms that form NO.

A neutral atom of Neon has an atomic number of 10, which means it has 10 protons and 10 electrons. We need to find which of the given ions also has 10 electrons. Magnesium (Mg) has an atomic number of 12, so a neutral atom has 12 electrons; the Mg²⁺ ion is formed when the atom loses 2 electrons. Therefore, the Mg²⁺ ion has 12 - 2 = 10 electrons, the same as Neon.

Common mistake: Students may mistakenly choose Ca²⁺ or K⁺, which have 18 electrons $(20-2=18 and 19-1=18 respectively)$, confusing the electron configuration with that of Argon instead of Neon.

Noble gases, found in Group 18 of the periodic table, are chemically inactive or inert because their outermost electron shell is completely full. This stable arrangement, with eight electrons in the outer shell (or two for Helium), is called an octet configuration. Since atoms react to achieve a stable electron configuration, and noble gases already have one, they have no tendency to gain, lose, or share electrons.

Common mistake: Students might be confused by the other options which describe shapes or use made-up terms, but the chemical reactivity of an element is determined by its electron configuration, not a physical shape.

The kinetic theory of matter states that particles (atoms, molecules, or ions) are in constant, random motion. Temperature is a measure of the average kinetic energy of these particles. Therefore, when the temperature of a substance is increased, its particles gain more energy, move faster, and their average kinetic energy increases.

Common mistake: A common mistake is to choose 'Decrease', which incorrectly suggests an inverse relationship, but temperature and kinetic energy are directly proportional.

Paramagnetism is a property of atoms that have unpaired electrons in their orbitals. Looking at the electronic configurations, Hydrogen (I) has one electron in its 1s orbital (1s¹), which is unpaired. For Oxygen (II), the 2p subshell has 4 electrons (2p⁴); when filling the three p-orbitals according to Hund's rule, two electrons will be left unpaired. Since both Hydrogen and Oxygen have unpaired electrons, they are both paramagnetic, making this pair the correct answer.

Common mistake: Students often mistakenly choose options with Zinc (IV), forgetting that its completely filled 3d¹⁰ and 4s² orbitals mean all its electrons are paired, making it diamagnetic, not paramagnetic.

According to the kinetic theory of gases, gas molecules are in a state of constant, random motion. As these molecules move around, they frequently hit the walls of the container they are in. The combined force of these countless collisions on the inner surface of the container is what creates the pressure exerted by the gas.

Common mistake: Students often mistakenly choose option B because they confuse collisions between gas molecules with the collisions against the container walls that actually cause pressure.