JAMB Chemistry 2001 Past Questions & Answers

This question is a direct application of Avogadro's Law. The law states that equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules. Since both gas X and gas Y have the same volume (25cm³), temperature, and pressure, they must contain the same number of molecules. Therefore, if gas X has Z molecules, gas Y will also have Z molecules.

Common mistake: Students might mistakenly think the number of molecules depends on the identity of the gas (Y), but Avogadro's law shows it only depends on volume, temperature, and pressure, which are the same for both gases.

First, write the balanced chemical equation for the reaction: 2H₂ + O₂ → 2H₂O. Next, calculate the number of moles of hydrogen gas (H₂) given, which is mass divided by molar mass (8.0g / 2g/mol) = 4 moles. From the equation, the mole ratio of hydrogen to water is 2:2 or 1:1, so 4 moles of hydrogen will produce 4 moles of water. Finally, find the mass of water produced by multiplying its moles by its molar mass (4 moles × 18g/mol), which gives 72.0g.

Common mistake: Students often make a mistake with the mole ratio, incorrectly thinking that 2 moles of H₂ produce 1 mole of H₂O, which would lead them to calculate 36.0g.

The identity of an element is determined by its number of protons. To find the charge of a particle, you compare the number of protons (positive charges) with the number of electrons (negative charges). In this case, there are 9 protons and 10 electrons, meaning there is one more electron than protons. This imbalance gives the particle an overall charge of -1, making it a negative ion.

Common mistake: Students may choose 'Neutral atom of a non metal' because 9 protons identify the element as Fluorine (a non-metal), but they forget to check if the number of electrons equals the number of protons to determine the charge.

The use of radiation to treat cancer is known as radiotherapy. This process requires radiation with very high energy and strong penetrating power to destroy cancerous cells deep inside the body. Gamma rays (ɣ-rays) are the most suitable for this because they are highly energetic and can penetrate tissues effectively to reach the tumour.

Common mistake: Students might confuse gamma rays with X-rays, as both are high-energy electromagnetic radiation used in medicine, but gamma rays are specifically noted for their use in radiotherapy to destroy cancerous growths.

First, find the relative molecular mass (RMM) of the oxide XO₂ using the formula RMM = 2 × Vapour Density, which gives 2 × 32 = 64. The RMM is also the sum of the atomic masses in the formula, so RMM of XO₂ = (Atomic mass of X) + 2 × (Atomic mass of O). By substituting the known values, we get the equation 64 = X + 2(16), which simplifies to 64 = X + 32, and solving for X gives an atomic mass of 32.

Common mistake: A common mistake is to confuse vapour density with relative molecular mass, which would lead to an incorrect calculation where RMM is taken as 32 instead of 64.

Robert Millikan's famous oil drop experiment led to the determination of the charge of a single electron. This was a crucial discovery because it allowed scientists to use J.J. Thomson's previously found charge-to-mass ratio to calculate the actual mass of an electron. In this way, Millikan's work was essential for confirming and establishing the definitive value of the charge-to-mass ratio.

Common mistake: The most common mistake is to choose option A; while it is true that Millikan determined the electron's charge, his work's full impact was realized when it was combined with the charge-to-mass ratio.

To identify the metal, we determine the electronic configuration of each element from its atomic number. Element Z has an atomic number of 20, giving it an electronic configuration of 2, 8, 8, 2. Metals are elements that tend to have few electrons (usually 1, 2, or 3) in their outermost shell, which they readily lose. Since Z has 2 electrons in its outermost shell, it is a metal.

Common mistake: Students might incorrectly choose Y (atomic number 16), failing to determine its electronic configuration (2, 8, 6), which shows it has 6 valence electrons and is therefore a non-metal.

The relative atomic mass of an element is the weighted average of the masses of its isotopes. To find the unknown mass 'm', we use the formula: [(% abundance of isotope 1 × mass 1) + (% abundance of isotope 2 × mass 2)] / 100. So, the equation is $[(90 × 16) + (10 × m)]$ / 100 = 16.2. Multiplying both sides by 100 gives 1440 + 10m = 1620, and solving for m shows that 10m = 180, which means m = 18.

Common mistake: Students often make calculation errors or incorrectly set up the weighted average formula, for example by forgetting to divide the sum of the products by 100.

The kinetic theory of gases states that the average kinetic energy of gas molecules is directly proportional to the absolute temperature. This means that when you heat a gas, you are giving its molecules more energy, which makes them move faster. Therefore, an increase in temperature leads to a direct increase in the average kinetic energy of the molecules.

Common mistake: Students often mistakenly choose option A because they associate high pressure with high energy, but pressure is caused by the force of collisions, while temperature is the direct measure of the average kinetic energy of the molecules.

Petroleum (oil) does not mix with water, so it forms a layer on the surface. Detergents act as emulsifying agents, meaning they have a part that attracts oil and another part that attracts water. This allows the detergent to break the large oil slick into tiny droplets that can be scattered and dispersed throughout the water, making it easier for natural processes to break them down.

Common mistake: Students might mistakenly choose 'Pouring organic solvents' because they know that oil dissolves in organic solvents, but this would only increase the overall pollution in the water.