JAMB Chemistry 2018 Past Questions & Answers

To solve this, first find the volume the gas would occupy at s.t.p. using the combined gas law, P₁V₁/T₁ = P₂V₂/T₂. Remember to convert the temperature to Kelvin $(T₁ = 273°C + 273 = 546 K)$. At s.t.p., P₂ = 1 atm and T₂ = 273 K, so the new volume V₂ = (P₁V₁T₂)/(T₁P₂) = $(4 × 10 × 273)$ / $(546 × 1)$ = 20 dm³. Finally, find the number of moles by dividing this volume at s.t.p. by the molar volume at s.t.p., which is n = 20 dm³ / 22.4 dm³/mol = 0.89 mol.

Common mistake: The most common mistake is forgetting to convert the initial temperature from Celsius (°C) to Kelvin (K) before calculation, which leads to an incorrect answer.

Charles' law states that the volume of a fixed mass of gas is directly proportional to its absolute temperature (in Kelvin). This means as you cool a gas, its volume decreases. If you plot a graph of volume against temperature and extend the line to where the volume would theoretically be zero, you reach a temperature called absolute zero. This temperature is 0 Kelvin, which is equivalent to -273°C.

Common mistake: Students often mistakenly choose 0°C because they confuse the zero point on the Celsius scale with absolute zero, which is the temperature required for gas law calculations.

Ethene is a plant hormone that is naturally produced by fruits to regulate their ripening process. Applying ethene gas to unripe fruits speeds up this process, making them ready for consumption faster. The other options, like propane, methane, and butane, are alkanes and do not have this biological effect on fruits.

Common mistake: Students may confuse ethene with other simple hydrocarbons like methane or propane, as they are all gases, but only ethene has the specific hormonal effect needed for fruit ripening.

This question tests knowledge of qualitative analysis, specifically the solubility of metal sulphides. While most metal sulphides react with and dissolve in dilute acids like HCl, the sulphides of Group II cations in qualitative analysis, such as Copper(II) sulphide (CuS), are exceptions. These sulphides are very insoluble and do not react with dilute non-oxidizing acids. The other options, FeS, ZnS, and Na₂S, all react with and dissolve in dilute HCl.

Common mistake: Students often remember that many metal sulphides are insoluble in water but forget the specific rule for solubility in dilute acid, leading them to incorrectly choose FeS or ZnS.

Sodium hydroxide (NaOH) is a strong base, so it completely breaks down in water to give hydroxide ions (OH⁻). The concentration of OH⁻ is the same as the solution's concentration, which is 0.001 mol dm⁻³ or 10⁻³ mol dm⁻³. First, we find the pOH by calculating -log[OH⁻], which gives pOH = -log(10⁻³) = 3. Since pH + pOH = 14, we can find the pH by subtracting the pOH from 14, so pH = 14 - 3 = 11.

Common mistake: A common mistake is to calculate the pOH as 3 and incorrectly choose that as the final pH, forgetting that the question asks for pH and that pH = 14 - pOH for a base.

In the complex ion [Cu(NH₃)₄]²⁺, the central copper ion (Cu²⁺) has empty orbitals and accepts pairs of electrons from the four surrounding ammonia (NH₃) molecules. Each ammonia molecule acts as a ligand, donating a lone pair of electrons from its nitrogen atom to form a bond with the copper ion. This type of covalent bond, where one atom provides both electrons for the shared pair, is specifically called a coordinate bond.

Common mistake: Students might mistakenly choose 'covalent' because a coordinate bond is a type of covalent bond, but 'coordinate' is the more specific and correct answer describing bonding in complex ions.

A chemical change is a process where a new substance is formed, and it is usually difficult to reverse. Rusting occurs when iron reacts with oxygen and water in the air to form a completely new substance, iron(III) oxide, which we see as rust. The other options are physical changes because no new substance is created; for example, melting ice is just water changing its state from solid to liquid.

Common mistake: Students often mistakenly choose the dissolution of salt in water, thinking a new substance is formed, but it is a physical change because the salt can be recovered by simply evaporating the water.

This question is solved using the combined gas law, which states (P₁V₁)/T₁ = (P₂V₂)/T₂. Let the initial conditions be P₁, V₁, and T₁ = 273 K. The final conditions are P₂ = 2P₁, V₂ = 2V₁, and we need to find T₂. Substituting into the equation gives (P₁V₁)/273 = (2P₁ * 2V₁)/T₂, which simplifies to $1/273$ = 4/T₂. Therefore, T₂ = 4 * 273 = 1092 K.

Common mistake: Students often mistakenly choose 546 K because they only double the initial temperature, forgetting that since both pressure and volume are doubled, the temperature must be multiplied by four.

The Kinetic Theory of Matter states that particles are in constant, random motion, and temperature is a measure of the average kinetic energy of these particles. Therefore, when the temperature is increased, the particles absorb this energy, causing them to move faster. This increase in the speed of the particles directly results in an increase in their kinetic energy.

Common mistake: Students might mistakenly choose 'remain constant' if they fail to recall the direct relationship between temperature and the average kinetic energy of particles as a core principle of the Kinetic Theory.

Sulphur is a highly flammable non-metal and is a key ingredient in the head of a matchstick. When a match is struck, the friction creates heat which easily ignites the sulphur. The burning sulphur then sets fire to the other chemicals in the match head, causing the wood to light up.

Common mistake: Students might mistakenly choose nitrogen because it is a common non-metal, but it is actually very unreactive and is not used for starting fires.