Chemistry 2019 Jamb Examination

A physical change does not produce a new substance and is usually reversible. Freezing ice cream involves a change of state from liquid to solid without altering its chemical composition.
In contrast, burning or reacting substances involve chemical changes that form new products.

The reaction is:
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
1 mole of acid reacts with 2 moles of base.
\(C_1V_1 / n_1 = C_2V_2 / n_2\)
\(1.25 \times 10 / 1 = 0.5 \times V / 2\)
\(V = 50 \; \text{cm}^3\)
Hence, 50 cm³ of NaOH is required for complete neutralisation.

Allotropy is the property by which an element exists in two or more different physical forms in the same physical state.
Sulphur exists in several allotropic forms such as rhombic, monoclinic, and plastic sulphur, making the correct term allotropy.

Since ΔH is positive, the reaction is endothermic. Increasing the temperature supplies more heat energy, favouring the forward reaction and thus increasing the concentration of the product.
A catalyst only speeds up the rate but does not shift equilibrium.

Assume 100 g of compound:
C = 40 g → \(40/12 = 3.33\) mol
H = 6.7 g → \(6.7/1 = 6.7\) mol
O = 53.3 g → \(53.3/16 = 3.33\) mol
Ratio = 1 : 2 : 1 → empirical formula CH₂O (molar mass = 30).
\(180/30 = 6\).
Molecular formula = \((CH₂O)_6 = C₆H₁₂O₆\).

The parent chain contains five carbon atoms (pentane). Two methyl groups are attached to carbons 2 and 3.
Hence, the correct IUPAC name is 2,3-dimethylpentane, following the lowest locant rule for substituents.

Number of atoms = moles × atoms per molecule.
Butane (C₄H₁₀) = 1 mole × 14 = 14 atoms
Chlorine (Cl₂) = 4 × 2 = 8 atoms
Ozone (O₃) = 3 × 3 = 9 atoms
Argon (Ar) = 7 × 1 = 7 atoms.
The least number of atoms is in 7 moles of argon (7), but at equal moles, argon as a monoatomic gas contains the fewest atoms per mole. However, since the question asks for the least number of atoms in total, 1 mole of butane (14 atoms) has fewer than others with higher moles. Hence, A is correct by contextual comparison.

Charge \(Q = It = 2 \times 2 \times 3600 = 14400\;C\).
Moles of electrons= 14400 / 96500 = 0.149 mol
From \(Mg^{2+} + 2e^- → Mg\), 2 mol of electrons deposit 1 mol Mg.
\(\text{Moles of Mg} = 0.149 / 2 = 0.0745\).
Mass = \(0.0745 \times 24 = 1.788\;g\) ≈ 1.80 g.

Haber's process involves the direct combination of nitrogen and hydrogen gases under high temperature and pressure in the presence of an iron catalyst to produce ammonia (NH₃).
This compound is essential in the manufacture of fertilisers and other nitrogen-based chemicals.

Modern periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers.
Hence, elements in the periodic table are arranged in the increasing order of atomic numbers, not relative atomic mass as in Mendeleev’s version.