Chemistry 2019 Jamb Examination

To find solubility, calculate the number of moles and divide by volume in dm³.
\(\text{Moles} = \dfrac{58.5}{111} = 0.527 \; \text{mol}\)
Volume = 250 cm³ = 0.25 dm³.
Concentration
\(= \dfrac{0.527}{0.25} = 2.108 \; \text{mol dm}^{-3}\).
Therefore, solubility ≈ 2.0 mol dm⁻³.

Ionisation energy and electron affinity both increase across a period due to increasing nuclear charge, which attracts electrons more strongly.
However, they decrease down a group because of increased atomic size and shielding effect.
Thus, properties ii and iv fit the description.

Water has a bent or V-shaped molecular geometry due to two lone pairs of electrons on oxygen causing bond pair repulsion.
The bond angle is approximately 105° instead of 109.5° as in a perfect tetrahedral structure.

The reaction between sodium carbonate and hydrochloric acid involves a strong acid and a weak base, producing an acidic solution.
The suitable indicator should have a transition point in the acidic range (around pH 3).
Therefore, methyl orange (pH ≈ 3) is appropriate.

After one half-life (20 years), half the particles remain.
After the second half-life (40 years), half of the remaining amount also decays.
\(N = 100,000 \times (\dfrac{1}{2})^2 = 25,000\) particles.
Thus, 25,000 particles remain after 40 years.

The fountain experiment demonstrates the high solubility of gases in water.
Ammonia (NH₃) and hydrogen chloride (HCl) dissolve readily in water, forming a striking fountain effect when the gas dissolves and water rushes in.
Hence, the correct pair is HCl and NH₃.

Chlorofluorocarbons (CFCs) release chlorine atoms when exposed to ultraviolet radiation in the stratosphere.
These chlorine atoms catalytically destroy ozone molecules, reducing the ozone concentration and leading to ozone layer depletion.

Acids react with carbonates or hydrogen carbonates to produce a salt, water, and carbon dioxide gas.
The general reaction is:
Acid + Carbonate → Salt + H₂O + CO₂
This behaviour characterises a chemical property of acids.

Air and bronze are mixtures without fixed compositions.
Air is a mixture of gases like nitrogen, oxygen, and carbon dioxide, while bronze is an alloy of copper and tin.
Since mixtures have variable composition, they cannot be represented by definite chemical formulas.

In dichromate ion (Cr₂O₇²⁻), oxygen has an oxidation number of -2. Let the oxidation number of chromium be x.
\(2x + (7 \times -2) = -2\) → \(2x - 14 = -2\) → \(x = +6\).
In Cr³⁺, chromium has +3. Thus, the oxidation number changes from +6 to +3, indicating reduction of chromium.