Chemistry 2014 Jamb Examination

pH is a measure of the hydrogen ion concentration in a solution and is defined mathematically as the negative logarithm of the hydrogen ion concentration. It expresses how acidic or basic a solution is.\[ pH = -\log_{10}[H^+] = \log_{10}\left(\dfrac{1}{[H^+]}\right) \]Since hydronium ions \(H_3O^+\) are the hydrated form of hydrogen ions, the formula can also be written as:\[ pH = \log_{10}\left(\dfrac{1}{[H_3O^+]}\right) \]This means that a lower concentration of hydrogen ions corresponds to a higher pH (more basic), while a higher concentration results in a lower pH (more acidic).

Potassium hydrogentetraoxosulphate (IV), commonly known as potassium bisulphite (KHSO3), is an acidic salt formed from a strong acid and a weak base. In aqueous solution, it releases hydrogen ions \(H^+\), making the solution acidic. This acidity causes blue litmus paper to turn red. In contrast, salts like sodium trioxocarbonate (IV) are basic and would turn red litmus blue instead.

Nitrogen obtained by fractional distillation of air is not completely pure; it contains small quantities of rare gases such as argon, neon, and krypton, which have higher densities than nitrogen. These impurities slightly increase the overall density of the nitrogen sample. In contrast, nitrogen obtained chemically from compounds is usually purer and therefore lighter.

Temporary hardness in water is caused by the presence of dissolved bicarbonates of calcium and magnesium. These salts decompose upon boiling to form insoluble carbonates that can be removed. The reaction involved is:\[ \text{Ca(HCO}_3)_2 \rightarrow \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \]Thus, water containing \(Ca(HCO_3)_2\) and \(Mg(HCO_3)_2\) is temporarily hard, as the hardness can be removed by boiling.

Efflorescence is the process by which a hydrated salt loses its water of crystallisation when exposed to air, forming an anhydrous or less hydrated compound. This occurs when the vapour pressure of the water in the hydrated salt is greater than that of the surrounding atmosphere. An example is washing soda \(Na_2CO_3·10H_2O\), which readily loses water to form \(Na_2CO_3·H_2O\) or anhydrous \(Na_2CO_3\).

To calculate solubility in mol dm^-3:
Calculate the molar mass of Pb(NO₃)₂\[ M = 207 + (14 \times 2) + (16 \times 6) \]\[= 331 \; \text{g mol}^{-1} \]Calculate moles of solute\[ n = \dfrac{16.55}{331} = 0.05 \; \text{mol} \]Convert solvent mass to volume
Assuming density of water = 1 g cm^-3,
100 g = 100 cm³ = 0.1 dm³
Calculate solubility\[ S = \dfrac{0.05}{0.1} = 0.5 \; \text{mol dm}^{-3} \]

An emulsion is a colloidal system formed when tiny droplets of one liquid are dispersed in another immiscible liquid. Examples include milk (fat dispersed in water) and butter (water dispersed in fat). The key feature is that both the dispersed phase and the dispersion medium are liquids.

Beryllium (Be) in Group 2 and aluminium (Al) in Group 13 exhibit a phenomenon known as diagonal relationship. This occurs because they have similar charge densities and polarising power due to comparable atomic and ionic sizes. As a result, both form covalent compounds, amphoteric oxides, and have similar chemical reactivities.

A difference in electronegativity of 3.0 indicates a very large disparity in the ability to attract electrons. This means one atom will completely transfer its electron(s) to the other, leading to the formation of an ionic bond. Typically, a difference greater than 1.7 signifies ionic character, whereas smaller differences correspond to covalent bonds.

For the element \( ^{60}_{27}\text{Co} \):
• Atomic number = 27 → Number of protons = 27
• Number of electrons = 27 (since the atom is neutral)
• Number of neutrons = Mass number − Atomic number\[ = 60 − 27 = 33 \]Therefore, cobalt has 27 protons, 33 neutrons, and 27 electrons.