2024 - JAMB Chemistry Past Questions and Answers - page 5

Oxidation is the loss of electrons or an increase in oxidation state. Therefore, when a species undergoes oxidation, its oxidation number increases.

Solution:

1. Calculate molar mass of NaCl: Na = 23 g/mol, Cl = 35.5 g/mol, Molar mass = 58.5 g/mol
2. Calculate number of moles: Moles = mass / molar mass = 127 / 58.5 ≈ 2.17 mol
3. Calculate solubility: Solubility = moles / volume = 2.17 mol / 1.0 dm³ = 2.17 mol·dm^-3
4. The closest option is 2.2 mol·dm^-3

The silica impurity (SiO₂) is removed by reacting it with a base such as sodium hydroxide (NaOH) to form soluble sodium silicate. This is part of the Bayer process.

Other options are incorrect because:

• Electrolysis: Used to extract aluminium from molten ore.
• Filtration: Removes solids but not the primary method for silica removal.
• Precipitation: Separates solids but not the main method for silica removal.

Water gas is produced by passing steam over red-hot coke, producing hydrogen gas (H₂) and carbon monoxide (CO).

Bitumen is a viscous, black liquid or semi-solid form of petroleum used as a binder in asphalt concrete for surfacing roads.

The compounds are:

• CH₃-CH₂-OH (ethanol)
• CH₃-O-CH₃ (dimethyl ether)

They have the same molecular formula (C₂H₆O) but different structural formulas, so they are isomers.

Calculation steps:

1. Molar mass of He ≈ 4 g/mol
2. Moles of He = 11.5 / 4 = 2.875 mol
3. Number of molecules = 2.875 × 6.022 × 10²³ ≈ 1.73 × 10²⁴ molecules

Recrystallization is the preferred method for obtaining highly pure solutes. It involves dissolving the impure solid in a suitable solvent and allowing pure crystals to form slowly, leaving impurities in solution. Other methods:

• Distillation: Separates liquids by boiling points
• Sieving: Separates solids by particle size
• Precipitation: Forms solid from solution but may not guarantee high purity

Fats and oils are triesters of glycerol and fatty acids. Glycerol is a trihydric alcohol with three hydroxyl groups.

The substance is water. It reacts with sodium to form sodium hydroxide (alkali) and changes white anhydrous CuSO₄ to blue hydrated CuSO₄·5H₂O.

Other substances like acids, bases, or solvents do not perform both reactions.