Chemistry 2013 Jamb Examination

Nitrogen makes up about 78% of the Earth's atmosphere and serves as a diluent for oxygen. Its presence reduces the concentration of oxygen, preventing spontaneous combustion and moderating the rate of oxidation and respiration processes. This balance ensures that living organisms can survive and that combustion does not occur uncontrollably in the atmosphere.

Anhydrous cobalt(II) chloride is blue in colour, but when exposed to steam or moisture, it absorbs water molecules to form cobalt(II) chloride hexahydrate, which is pink. This colour change from blue to pink is a reversible reaction and is commonly used as a qualitative test for the presence of water or moisture in substances.

Hygroscopic substances are those that absorb moisture from the air without forming a solution. Calcium chloride \(\text{CaCl}_2\) is strongly hygroscopic because it readily attracts and retains water molecules from its surroundings to form hydrated salts. This property makes it useful as a drying agent or desiccant in laboratories and industrial applications.

Impurities disrupt the orderly arrangement of particles in a pure substance, weakening the forces holding them together. As a result, less energy is required to break these forces, leading to a decrease in the melting point. This principle is often used in identifying the purity of a substance — a sharp melting point indicates purity, while a lowered or broadened range suggests impurities.

The balanced chemical equation for the reaction is:\[ 2C + O_2 \rightarrow 2CO \]From the equation, 1 mole of \(O_2\) produces 2 moles of \(CO\). Therefore, the volume of \(CO\) produced is twice the volume of \(O_2\) used.
Given that \(O_2 = 10\;\text{dm}^3\):\[ \text{Volume of } CO = 2 \times 10 = 20\;\text{dm}^3 \]Thus, 20 dm³ of carbon(II) oxide is formed when 10 dm³ of oxygen reacts with excess carbon.

According to Graham’s Law of Diffusion:\[ \dfrac{r_Y}{r_Z} = \sqrt{\dfrac{M_Z}{M_Y}} \]Given that \(r_Y = 2r_Z\) and \(M_Y = 64\):\[ 2 = \sqrt{\dfrac{M_Z}{64}} \]Squaring both sides:\[ 4 = \dfrac{M_Z}{64} \]\[ M_Z = 256 \]This result appears inconsistent with the expected trend, as a heavier gas should diffuse slower. Reversing the ratio to represent Z diffusing slower gives:\[ \dfrac{r_Z}{r_Y} = \sqrt{\dfrac{M_Y}{M_Z}} \]\[ \dfrac{1}{2} = \sqrt{\dfrac{64}{M_Z}} \]Squaring both sides:\[ \dfrac{1}{4} = \dfrac{64}{M_Z} \]\[ M_Z = 256 \]This means gas Z has a molecular mass of 256 g/mol, which aligns with a slower diffusion rate under identical conditions.

Cobalt-60 is a strong gamma-ray emitter and is widely used in industrial radiography for detecting internal flaws in metal welds and castings. Its high-penetration gamma radiation allows engineers to inspect metal components without cutting or damaging them, ensuring structural integrity in pipelines, aircraft, and pressure vessels.

Carbon dioxide \(\text{CO}_2\) consists of a carbon atom doubly bonded to two oxygen atoms (O=C=O). The molecule has two regions of electron density around the central carbon atom, which repel each other to form a bond angle of 180°, resulting in a linear geometry. This arrangement minimises electron repulsion according to the Valence Shell Electron Pair Repulsion (VSEPR) theory, giving \(\text{CO}_2\) its linear shape.