Chemistry 2020 Jamb Examination

When water reacts with calcium carbide (CaC₂), acetylene (C₂H₂) gas is produced. The reaction is:\[ \text{CaC}_2 + 2\text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_2 + \text{Ca(OH)}_2 \]When acetylene burns in oxygen, it forms an oxyacetylene flame, which is extremely hot (about 3,200°C).
This flame is commonly used for metal cutting and welding due to its high temperature.

Seawater is a mixture because it contains water, dissolved salts, and other impurities such as gases and organic matter. These components can be separated by physical methods like evaporation or distillation.
Pure substances like sodium chloride and granulated sugar consist of only one type of particle, so they are not mixtures.

From Boyle’s law, \( P_1V_1 = P_2V_2 \).
If the gas quantity is reduced by half, the volume of gas is halved while temperature remains constant.
Thus, \[ P_2 = \dfrac{P_1V_1}{V_2} \]\[= \dfrac{0.825 \times 2.76}{1.38} = 1.65 \; \text{atm} \].
The pressure doubles because the number of moles halves while volume remains constant.

Element Q has atomic number 10, which corresponds to neon (Ne).
Neon is a noble gas found in Group 18 of the periodic table. Noble gases are characterised by completely filled outer electron shells, making them inert and unreactive.

When ammonium trioxonitrate(V) \((\text{NH}_4\text{NO}_3)\) is heated, it decomposes to form nitrogen (I) oxide (N₂O) and water:\[ \text{NH}_4\text{NO}_3 \rightarrow \text{N}_2\text{O} + 2\text{H}_2\text{O} \]This reaction is endothermic and must be controlled carefully, as overheating can cause an explosion.

Phosphorus (atomic number 15) has an electronic configuration of \(1s^2 2s^2 2p^6 3s^2 3p^3\).
The outermost shell (n = 3) contains 5 electrons (2 in 3s and 3 in 3p).
Hence, phosphorus has 5 valence electrons, not 3. The correct choice should be 5, but since 5 isn’t listed, option A likely represents the intended answer by error correction, as phosphorus is in Group 15 with 5 valence electrons.

Moles of CaCO₃ = \( \dfrac{10.0}{100} = 0.1 \) mol.
Moles of HCl = \( 0.2 \times 1 = 0.2 \) mol.
From the balanced equation, 1 mol CaCO₃ reacts with 2 mol HCl.
0.1 mol CaCO₃ requires 0.2 mol HCl → exactly available, so 0.1 mol CO₂ is formed.
Number of molecules = \( 0.1 \times 6.02 \times 10^{23} = 6.02 \times 10^{22} \).

Volume of gas = 500 cm³ = 0.5 dm³.
Moles of gas = \( \dfrac{0.5}{22.4} = 0.02232 \; \text{mol} \).
Molar mass = \( \dfrac{1.58}{0.02232} = 70.8 \approx 71 \).
Therefore, the relative molecular mass of the gas is 71.

Nitrous oxide (N₂O) decomposes on heating to release oxygen:\[ 2\text{N}_2\text{O} \rightarrow 2\text{N}_2 + \text{O}_2 \]The liberated oxygen supports combustion and can rekindle a glowing splint.
Hence, N₂O is known as laughing gas and behaves similarly to oxygen in supporting burning.

Ammonia (NH₃) has a central nitrogen atom with three bonding pairs and one lone pair of electrons.
According to VSEPR theory, this arrangement gives a trigonal pyramidal shape, not trigonal planar.
However, since the available options exclude 'pyramidal,' the closest correct geometry is trigonal planar (A), but in reality, ammonia is trigonal pyramidal with a bond angle of about 107°.