2025 - JAMB Chemistry Past Questions and Answers - page 7

The relationship between number of moles, mass, and molar mass is given by:

\[ n = \frac{m}{M} \]

Rearranging the formula to make molar mass the subject gives:

\[ M = \frac{m}{n} \]

The given mass is 44 g and the number of moles is 0.5 mol. Substituting these values:

\[ M = \frac{44}{0.5} \]

Dividing by 0.5 is equivalent to multiplying by 2:

\[ M = 44 \times 2 \]

\[ M = 88 \text{ g/mol} \]

Thus, the molecular mass of the alkanoic acid is 88 g/mol.

The rate of diffusion of gases depends on Graham’s law, which states that the rate of diffusion is inversely proportional to the square root of molar mass. This means heavier gases diffuse more slowly, while lighter gases diffuse faster.

The molar masses of the gases in option B are calculated as follows:

SO\(_2\): S = 32 and O = 16, so molar mass = 32 + (2 × 16) = 64 g/mol

O\(_2\): molar mass = 2 × 16 = 32 g/mol

NH\(_3\): N = 14 and H = 1, so molar mass = 14 + (3 × 1) = 17 g/mol

H\(_2\): molar mass = 2 × 1 = 2 g/mol

Since diffusion rate increases as molar mass decreases, the correct order from slowest to fastest diffusion is from highest molar mass to lowest molar mass:

\[ 64 > 32 > 17 > 2 \]

This corresponds to SO\(_2\), O\(_2\), NH\(_3\), H\(_2\).

Therefore, this is the correct order of increasing rate of diffusion.

Alkenes are hydrocarbons that contain at least one carbon–carbon double bond. In saturated hydrocarbons such as alkanes, the general formula is C\(_n\)H\(_{2n+2}\).

When a double bond is present, two hydrogen atoms are lost compared to the alkane. This reduces the number of hydrogen atoms by 2.

Therefore, the general formula becomes:

\[ C_nH_{2n} \]

This formula applies to all alkenes with one double bond. For example, when n = 2:

\[ C_2H_4 \]

This compound is ethene, the simplest alkene.

Thus, the general molecular formula of alkenes is C\(_n\)H\(_{2n}\).

The structure contains a carbon–carbon triple bond, so it is an alkyne. The longest continuous chain of carbon atoms must first be identified. Counting along the chain that includes the triple bond gives five carbon atoms, so the parent name is pent.

The triple bond must be assigned the lowest possible number. Numbering from the end closest to the triple bond places it between carbon 2 and carbon 3, so the parent name becomes pent-2-yne.

There is also a methyl group attached to carbon 4 of this chain. This substituent is therefore named 4-methyl.

Combining the substituent and the parent name gives the full IUPAC name as 4-methylpent-2-yne.

Therefore, the correct name is 4-methylpent-2-yne.

Extraction processes are required when metals occur in nature as compounds such as oxides or sulphides. In such cases, chemical methods are needed to separate the metal from other elements.

Gold, however, is extremely unreactive. Because of this low reactivity, it does not readily combine with oxygen or other elements.

As a result, gold is commonly found in its pure metallic state in the earth’s crust. Metals that occur naturally in their pure state are described as being free in nature.

Since gold already exists in its elemental form, no chemical extraction is required.

Therefore, gold does not require extraction because it is free in nature.

Magnesium tetraoxosulphate(VI) is the chemical name for magnesium sulphate, also known as Epsom salt.

When taken into the body, this substance increases the amount of water in the intestines. The increased water content softens the stool and makes it easier to pass.

This effect helps relieve constipation by promoting bowel movement. Substances that help stimulate bowel movement in this way are called laxatives.

Therefore, magnesium tetraoxosulphate(VI) is commonly used as a laxative.

The graph shows how solubility varies with temperature for substances X, Y, and Z. Observing the line for substance Y, it slopes upward from left to right.

An upward slope indicates that as temperature increases, the solubility value also increases. This means more of substance Y dissolves at higher temperatures.

The increase occurs continuously without decreasing or remaining constant, showing a steady rise.

Therefore, the correct conclusion is that the solubility of Y increases steadily as temperature increases.

Stainless steel contains chromium, which forms a protective layer that prevents rusting. This makes the alloy highly resistant to corrosion.

It is also strong, durable, and does not react easily with food substances. These properties make it suitable for making utensils used in food preparation and eating.

Because of its resistance to rust and its hygienic nature, stainless steel is widely used to manufacture items such as spoons, knives, and forks.

Therefore, stainless steel alloy is commonly used for making cutlery.

The spontaneity of a reaction depends on the Gibbs free energy change, given by the expression:

\[ \Delta G = \Delta H - T\Delta S \]

A reaction is spontaneous when ΔG is negative.

In this case, ΔH is positive but small, while ΔS is positive and large. The term TΔS therefore becomes a large positive value.

Since this large positive quantity is subtracted from the small positive ΔH, the overall result becomes negative:

\[ \Delta G < 0 \]

A negative Gibbs free energy change means the reaction can occur without external energy input.

Therefore, the reaction is spontaneous.

When the balloon is placed in a freezer, the temperature of the gas inside decreases. According to the kinetic theory of gases, temperature is directly related to the average kinetic energy of gas particles.

As temperature decreases, the kinetic energy of the gas molecules also decreases. This causes the molecules to move more slowly.

Because the molecules move more slowly, they collide with the walls of the balloon less frequently and with less force. Pressure exerted by a gas depends on these collisions.

As the internal pressure drops, the external atmospheric pressure becomes greater than the pressure inside the balloon. This external pressure compresses the balloon, reducing its volume.

Therefore, the balloon shrinks because gas motion reduces and exerts less pressure.