Chemistry 2015 Jamb Examination

In Cu₂S, each Cu atom has an oxidation number of +1 because sulphur is -2. In the products, Cu is in its elemental form with an oxidation number of 0. Therefore, the oxidation number of Cu decreases from +1 to 0, indicating a reduction of copper ions to metallic copper.

C₆H₁₄ (hexane) has five structural isomers:
1. n-Hexane
2. 2-Methylpentane
3. 3-Methylpentane
4. 2,2-Dimethylbutane
5. 2,3-Dimethylbutane
Each has the same molecular formula but different arrangements of carbon atoms, giving rise to five distinct isomers.

Matter is composed of atoms, ions, and molecules depending on its physical or chemical state. Salt is a compound composed of ions and is not a fundamental particle of matter. Hence, it does not qualify as a basic constituent of matter.

A bomb calorimeter is used to determine the heat of combustion of fuels like petrol. It measures the energy released when a known quantity of the fuel is burnt in oxygen within a sealed container, allowing the calculation of its energy value. The other devices serve different purposes in refining or temperature measurement.

The balanced chemical equation is:
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
Moles of NaOH = \( M_1V_1 = 0.1 \times \dfrac{20}{1000} = 0.002 \; mol \)
From the equation, 1 mole of H₂SO₄ neutralises 2 moles of NaOH.
Therefore, moles of H₂SO₄ required = \( \dfrac{0.002}{2} = 0.001 \; mol \)
Given concentration of H₂SO₄ = 0.5 mol dm^-3
Volume = \( \dfrac{n}{C} = \dfrac{0.001}{0.5} = 0.002 \; dm^3 = 2.0 \; cm^3 \)
Hence, 2.0 cm³ of 0.5 mol dm⁻³ H₂SO₄ will exactly neutralise 20 cm³ of 0.1 mol dm⁻³ NaOH solution.

Assume 100 g of the compound:
Carbon = 40.0 g, Hydrogen = 6.7 g, Oxygen = 53.3 g.
Number of moles of each element:\[ \text{C: } \dfrac{40.0}{12} = 3.33 \quad\]\[\text{H: } \dfrac{6.7}{1} = 6.7 \quad \]\[ \text{O: } \dfrac{53.3}{16} = 3.33 \]Divide by the smallest value (3.33):
C : H : O = 1 : 2 : 1
Empirical formula = CH₂O.
Empirical formula mass = (12) + (2 × 1) + (16) = 30.\[ \text{n} = \dfrac{\text{Molecular mass}}{\text{Empirical formula mass}} \]\[= \dfrac{180}{30} = 6 \]Therefore, molecular formula = (CH₂O) × 6 = C₆H₁₂O₆.

Let the oxidation state of Cr be \(x\). The sum of oxidation numbers in K₂Cr₂O₇ is zero.\[ 2(+1) + 2x + 7(-2) = 0 \]\[ 2 + 2x - 14 = 0 \]\[ 2x = 12 \]\[ x = +6 \]Hence, the oxidation state of chromium in potassium dichromate (K₂Cr₂O₇) is +6.

The third period of the periodic table includes elements with electrons in the third shell (n = 3). These are sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorus (P), sulphur (S), chlorine (Cl), and argon (Ar). Therefore, Na, Mg, S, and Ar correctly represent elements from the third period.

Dissolving sugar in tea increases the randomness or disorder of the system, as the sugar molecules move from an ordered crystalline structure to a dispersed state within the liquid. This results in an increase in entropy, which corresponds to a positive entropy change. However, since the process is spontaneous, it involves an increase, not a decrease, in entropy.

Two gaseous molecules combine to form a solid (ammonium chloride). The process involves a decrease in the number of free-moving particles and a reduction in randomness or disorder. Hence, the system becomes more ordered, resulting in a negative entropy change.