(a) Explain briefly the observations in each of the following processes: (i) when carbon (IV) oxide is bubbled through lime water, It turns milky but the mi...

(a)(i) CO₂ through lime water: at first, insoluble white calcium trioxocarbonate(IV) is formed, making the lime water milky:

\[ \text{Ca(OH)}_2 + \text{CO}_2 \to \text{CaCO}_3\downarrow + \text{H}_2\text{O} \]

On continued bubbling, excess CO₂ converts the insoluble CaCO₃ into soluble calcium hydrogentrioxocarbonate(IV), so the milkiness disappears (the solution becomes clear):

\[ \text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \to \text{Ca(HCO}_3)_2 \]

(a)(ii) Calcium hydroxide is a base but not amphoteric, so it does not react with (dissolve in) excess NaOH and remains as a precipitate. Lead(II) hydroxide is amphoteric; it reacts with excess NaOH to form the soluble plumbite (tetrahydroxoplumbate(II)) ion, so it dissolves:

\[ \text{Pb(OH)}_2 + 2\text{OH}^- \to \text{Pb(OH)}_4^{2-} \]

(b)(i) Primary standard solution: a solution of a substance of known high purity, stable in air, of accurately known concentration prepared by weighing the solute directly (it does not absorb moisture or CO₂ and has a high, well defined molar mass).

(b)(ii) Mass of Na₂CO₃ for 250 cm³ of 0.15 mol dm^-3:

Molar mass Na₂CO₃ \(= (2\times23) + 12 + (3\times16) = 106\ \text{g mol}^{-1}\).

\[ \text{moles} = 0.15 \times \frac{250}{1000} = 0.0375\ \text{mol} \] \[ \text{mass} = 0.0375 \times 106 = 3.98\ \text{g} \]

(c) Gas collection:

• (i) By upward displacement of air (denser than air): carbon(IV) oxide, CO₂ (or chlorine, HCl).
• (ii) By downward displacement of air (less dense than air): ammonia, NH₃ (or hydrogen).