Chemistry 2020 Jamb Examination

Adding concentrated hydrochloric acid \( \text{(HCl)} \) introduces more hydrogen ions \( \text{(H}^+\text{)} \) into the solution, increasing its acidity. pH is inversely related to the concentration of hydrogen ions; hence, as \( [\text{H}^+] \) increases, pH decreases.
Therefore, the resulting mixture will have a pH value less than 3.4, indicating a stronger acid.

Chromatography works by exploiting the difference in how substances move through a medium. The components of a mixture travel at different rates through a stationary phase under the influence of a mobile phase.
These varying rates of migration cause the components to separate effectively. Hence, chromatography separates substances based on their rates of migration.

Deliquescent substances are materials that absorb moisture from the atmosphere until they dissolve in the absorbed water and form a solution. They are a subset of hygroscopic substances, which can absorb water vapour but do not necessarily dissolve in it.
Hence, deliquescent substances are also described as hygroscopic in nature.

Helium \( \text{(He)} \) is a noble gas that is chemically inert and lighter than air. Its non-combustible nature makes it a safer alternative to hydrogen, which is flammable. Additionally, its low density allows balloons filled with helium to float and rise easily.
Therefore, helium is used because it is light and non-combustible.

The molar mass of \( \text{CuSO}_4 \) is approximately 160 g/mol. The number of moles in 32 g is:\[ n = \dfrac{32}{160} = 0.2 \; \text{mol} \]If 0.2 mol releases 13.0 kJ, then the heat of solution per mole is:\[ \dfrac{13.0}{0.2} = 65.0 \; \text{kJ/mol} \]Thus, the heat of solution of copper(II) tetraoxosulphate(VI) is 65.0 kJ/mol.

Assume the formula is \( \text{MCl}_x \). The percentage of M is given by:\[ \dfrac{27}{27 + 35.5x} \times 100 = 20.22 \]Solving for \( x \):
27 = 0.2022(27 + 35.5x) → 27 = 5.46 + 7.18x → x ≈ 3
Therefore, the compound is MCl₃.

From the electrolysis reaction at the anode:\[ 2H_2O \rightarrow O_2 + 4H^+ + 4e^- \]This shows that 4 moles of electrons (4 Faradays) are required to produce 1 mole of \( O_2 \).
For 0.125 mole of \( O_2 \):\[ 0.125 \times 4 = 0.5 \; \text{mol e}^- \]1 Faraday = 96,500 C\[ 0.5 \times 96,500 = 48,250 \; \text{C} \]Thus, the quantity of electricity required is 48,250 C.

Metallic bonding occurs between atoms of the same metal, involving a sea of delocalised electrons surrounding positive metal ions. It does not lead to the formation of new substances; instead, it gives metals their characteristic properties such as conductivity and malleability.
Covalent, coordinate, and ionic (electrovalent) bonding involve the combination of different atoms to form compounds, hence new substances. Therefore, metallic bonding does not form new substances.

Hydrogen cyanide and hydroxylamine react with carbonyl compounds such as aldehydes (alkanals) and ketones (alkanones) to form addition products. However, alkenes lack the polar carbonyl group necessary for such nucleophilic additions.
Therefore, an alkene does not undergo reactions with hydrogen cyanide or hydroxylamine.

Vulcanisation is the process of heating raw rubber with sulphur to improve its elasticity, strength, and resistance to wear. Sulphur atoms form cross-links between the long polymer chains of rubber, stabilising the material.
Hence, sulphur is the key substance used for the vulcanisation of rubber.