(a) Define the following terms:(i) Saturated solution; (ii) Solubility. (b) In an experiment to determine the solubility of a given salt Y, the following da...

(a) Definitions

• (i) Saturated solution: a solution that contains as much dissolved solute as it can hold at a given temperature in the presence of undissolved solute.
• (ii) Solubility: the maximum mass (or moles) of a solute that dissolves in a fixed volume (1 dm³) or 100 g of solvent to form a saturated solution at a given temperature.

(b) Solubility of salt Y

Mass of salt Y \(= 9.30 - 7.16 = 2.14\text{ g}\)

Mass of saturated solution \(= 17.85 - 7.16 = 10.69\text{ g}\)

Since density \(= 1.00\text{ g cm}^{-3}\), volume of solution \(= 10.69\text{ cm}^3 = 0.01069\text{ dm}^3\)

(i) \[\text{Solubility} = \frac{2.14}{0.01069} = 200\text{ g dm}^{-3}\]

(ii) \[\text{Solubility} = \frac{200}{100} = 2.0\text{ mol dm}^{-3}\]

(c) Bond broken on melting

• (i) NaCl(s): ionic (electrovalent) bond
• (ii) CO₂(s): weak van der Waals (intermolecular) forces
• (iii) SiO₂(s): giant covalent bonds
• (iv) Al(s): metallic bond

(d) Explanations

• (i) Down the group of alkali metals the atomic radius increases and the outer electron is more shielded from the nucleus, so ionisation energy falls and the single valence electron is lost more easily, increasing reactivity.
• (ii) Magnesium provides two delocalised electrons per atom (Na provides one) and has a smaller ion, so its metallic bonding is stronger, giving a higher melting point.
• (iii) Potassium has a larger atomic radius and lower ionisation energy than sodium, so it loses its outer electron more readily and is a better reducing agent.

(e) Isotopes

• (i) Isotopes are atoms of the same element having the same atomic number (proton number) but different mass numbers (different numbers of neutrons).
• (ii) \[A_r = \frac{(6\times 2)+(7\times 25)}{2+25} = \frac{12+175}{27} = \frac{187}{27} = 6.93\]