2025 - JAMB Chemistry Past Questions and Answers - page 5

Brine is a concentrated aqueous solution of sodium chloride. During electrolysis, reduction takes place at the cathode. In the solution, both sodium ions and water molecules are present, but water is reduced more readily than sodium ions. The reduction of water produces hydrogen gas and hydroxide ions according to the equation \\[2H_{2}O + 2e^{-} \\rightarrow H_{2} + 2OH^{-}\\]. This shows that hydrogen gas is formed at the cathode.

Therefore, the gas produced at the cathode is hydrogen.

An alkanoic acid is a saturated carboxylic acid with the general molecular formula

\[C_nH_{2n}O_2\]

Given that the empirical formula is

\[CH_2O\]

The molecular formula is always a whole-number multiple of the empirical formula.

Therefore, the molecular formula can be written as

\[(CH_2O)_x\]

Expanding this expression gives

\[C_xH_{2x}O_x\]

Since an alkanoic acid must also follow the general formula

\[C_nH_{2n}O_2\]

Compare the number of oxygen atoms in both formulas:

\[O_x = O_2\]

Therefore,

\[x = 2\]

Substituting \( x = 2 \) into the empirical formula multiple gives

\[(CH_2O)_2 = C_2H_4O_2\]

Thus, the molecular formula is

\[C_2H_4O_2\]

The relative molecular mass is calculated using the atomic masses:C = 12, H = 1, O = 16

\[\text{Relative molecular mass} = (2 \times 12) + (4 \times 1) + (2 \times 16)\]

\[= 24 + 4 + 32\]

\[= 60\]

Therefore, the relative molecular mass is

\[60\]

Cell notation is written with the oxidation half-cell on the left and the reduction half-cell on the right.

Magnesium is converted to magnesium ions according to the equation

\[ Mg \rightarrow Mg^{2+} + 2e^- \]

This shows that magnesium loses electrons. Loss of electrons is oxidation, so oxidation occurs at the magnesium electrode. This half-cell is written as

\[ Mg \;|\; Mg^{2+} \]

Lead ions gain electrons to form solid lead according to the equation

\[ Pb^{2+} + 2e^- \rightarrow Pb \]

This shows that reduction occurs at the lead electrode. This half-cell is written as

\[ Pb^{2+} \;|\; Pb \]

The salt bridge separating the two half-cells is represented by a double vertical line

\[ || \]

Combining the oxidation half-cell, the salt bridge, and the reduction half-cell gives the complete cell notation:

\[ Mg \;|\; Mg^{2+} \;||\; Pb^{2+} \;|\; Pb \]

Covalent molecular substances consist of individual molecules held together by intermolecular forces. 

These forces, such as van der Waals forces, are relatively weak compared to the strong covalent bonds within each molecule. 

During melting or boiling, the molecules move apart from one another, and only these weak intermolecular forces need to be overcome. Because only a small amount of energy is required to separate the molecules, the melting and boiling points are low. 

Therefore, the correct explanation is that they have weak intermolecular forces.

Solubility is defined as the mass of solute dissolved in 100 g of solvent. The total mass of the saturated solution is 12.5 g, and the mass of the dissolved salt obtained after evaporation is 2 g. The mass of water present in the solution is found by subtracting the mass of the salt from the total mass of the solution, which gives \\[12.5 - 2 = 10.5 \\text{ g}\\]. The solubility in grams per 100 g of water is calculated using the proportion \\[\\frac{2}{10.5} \\times 100\\]. This gives \\[\\frac{200}{10.5} = 19.05\\].

Therefore, the solubility is approximately 19.05 g per 100 g of water.

A reducing agent is a substance that donates electrons to another substance. When magnesium reacts with hydrochloric acid, magnesium atoms lose electrons to form Mg^{2+} ions. This can be represented as \\[Mg \\rightarrow Mg^{2+} + 2e^{-}\\]. Since magnesium loses electrons, it undergoes oxidation. A substance that undergoes oxidation causes another substance to be reduced, so it is called the reducing agent.

Therefore, magnesium acts as the reducing agent because it is oxidized.

A suitable packaging metal must be easy to shape, resistant to corrosion, and safe for containing food or drinks. Aluminium has these properties because it is lightweight, can be rolled into thin sheets, and forms a protective oxide layer that prevents corrosion. This makes it ideal for use in cans and foils.

Therefore, aluminium is the metal commonly used for packaging.

In the equation, element X reacts with hydrochloric acid to form XCl. Chlorine forms a chloride ion with a charge of −1, so the formula XCl shows that X must form a +1 ion. Elements that form +1 ions belong to Group 1 of the periodic table. Among the options given, potassium is a Group 1 element and forms KCl.

Therefore, X is potassium.

Reforming is a refining process in which the structure of hydrocarbons is changed to improve fuel quality, often by forming aromatic compounds from cyclic hydrocarbons. Cyclohexane can lose hydrogen atoms and rearrange to form benzene. This transformation produces benzene and hydrogen gas, which is characteristic of reforming.

Therefore, the conversion of cyclohexane to benzene represents reforming.

To determine the number of lone pairs, the valence electrons on the central atom of each molecule must be considered. In H₂O, oxygen has 6 valence electrons and forms two bonds with hydrogen, leaving 4 electrons, which make 2 lone pairs. In NH₃, nitrogen has 5 valence electrons and forms three bonds, leaving 2 electrons, which form 1 lone pair. In CH₄, carbon has 4 valence electrons and forms four bonds, leaving no lone pairs. In CO₂, carbon forms two double bonds with oxygen, using all its valence electrons, so carbon has no lone pairs. Comparing these, water has the highest number of lone pairs, which is 2 lone pairs.

Therefore, the correct answer is H₂O.