Chemistry 2025 Jamb Examination

To calculate solubility in grams per 100g of water, we first find the mass of the solvent (water) in the solution.\[\text{Mass of water} = \text{Mass of solution} - \text{Mass of solute}\]\[\text{Mass of water} = 12.5\text{g} - 2\text{g} = 10.5\text{g}\]Solubility is the maximum mass of solute that can dissolve in 100g of water at a specific temperature:\[\text{Solubility} = \frac{\text{Mass of solute}}{\text{Mass of solvent}} \times 100\]\[\text{Solubility} = \frac{2}{10.5} \times 100 \approx 19.05\text{g}\]Rounding to the nearest whole number provided in the options, the value is approximately 20.0.

This reaction involves an alkane (propane) reacting with a halogen (chlorine). In the presence of ultraviolet light, a hydrogen atom in the propane molecule is replaced by a chlorine atom. This type of reaction, where one atom or group of atoms is replaced by another, is known as a substitution reaction. Alkanes typically undergo substitution because they are saturated hydrocarbons.

Brine is a concentrated aqueous solution of sodium chloride (\(NaCl\)). During electrolysis, the ions present are \(Na^+\), \(Cl^-\), \(H^+\), and \(OH^-\). At the cathode (negative electrode), the cations \(Na^+\) and \(H^+\) are attracted. Since hydrogen is lower in the electrochemical series than sodium, \(H^+\) ions are preferentially discharged to form hydrogen gas (\(H_2\)).

A salt forms a neutral solution (pH 7) if it is derived from a strong acid and a strong base. Sodium chloride (\(NaCl\)) is formed from hydrochloric acid (a strong acid) and sodium hydroxide (a strong base). Because neither ion undergoes significant hydrolysis in water, the concentration of \(H^+\) and \(OH^-\) ions remains balanced, resulting in a neutral pH. Other salts like ammonium chloride are acidic, while sodium trioxocarbonate(IV) is basic.

For a reaction to be spontaneous, the Gibbs free energy change (\(\Delta G\)) must be negative, where \(\Delta G = \Delta H - T\Delta S\). In this reaction, a gas (\(PCl_5\)) produces another gas (\(Cl_2\)) and a solid, but generally, the decomposition of a molecule into multiple parts increases entropy (\(\Delta S\) is positive). If the reaction is exothermic (\(\Delta H\) is negative), then \(\Delta G\) will definitely be negative at all temperatures, making the reaction spontaneous.

This reaction shows an alkanol (ethanol) reacting with an alkanoic acid (ethanoic acid) to produce an ester (ethyl ethanoate) and water. The process of forming an ester from an alcohol and an organic acid is specifically known as esterification. It is often catalysed by concentrated sulphuric acid.

The structure provided is ethyl propanoate, an ester. Esters are named after the alkanol part first (ethyl) and the alkanoic acid part second (propanoate). The 'ethyl' part (\(-CH_2CH_3\)) comes from ethanol, and the 'propanoate' part (\(CH_3CH_2COO-\)) comes from propanoic acid. Therefore, the reactants used to create this ester are propanoic acid and ethanol.

Stainless steel is an alloy of iron, chromium, and nickel. It is highly resistant to corrosion and staining, which makes it an ideal material for items that come into contact with water and food, such as cutlery, surgical instruments, and kitchen sinks. Bridges usually use carbon steel, and rock crushers often use manganese steel for its hardness.

Ammoniacal copper(I) chloride (or sometimes copper(II) chloride in specific industrial gas analysis) is a standard reagent used to absorb Carbon(II) oxide (carbon monoxide). Both producer gas and water gas contain high percentages of \(CO\). The reagent forms a complex with the carbon monoxide, allowing it to be separated from other gases like hydrogen or nitrogen.

Water molecules contain oxygen bonded to hydrogen. Because oxygen is highly electronegative, it creates a strong dipole. The attraction between the partially positive hydrogen of one molecule and the lone pair of oxygen on another molecule is called a hydrogen bond. These intermolecular forces are much stronger than typical Van der Waals forces, providing enough cohesion to keep water in a liquid state at room temperature, unlike similar-sized molecules like \(H_2S\).