iGCSE Chemistry (0620) Theory (Core):12.5 Identification of ions and gases: Exam Style Questions Paper 3- New Syllabus

(a)(i) sodium

From Table 3.1, the sodium ion ($\mathrm{Na}^+$) has the highest mass (8.3 mg) among all the positive ions listed, indicating the highest concentration in this sample.

(a)(ii) $\mathrm{SO}_4^{2-}$

The sulfate ion consists of one sulfur atom bonded to four oxygen atoms, carrying an overall 2- charge.

(a)(iii)

test: (dilute nitric acid and aqueous) silver nitrate
observations: yellow precipitate

The sample is first acidified with dilute nitric acid to remove ions like carbonates that might interfere. Aqueous silver nitrate is then added. The silver ions react with iodide ions to form a bright yellow precipitate of silver iodide: $\mathrm{Ag}^+(\mathrm{aq}) + \mathrm{I}^-(\mathrm{aq}) \rightarrow \mathrm{AgI}(\mathrm{s})$.

(a)(iv) mass = 1.9 mg

The mass in $1000\text{ cm}^3$ is $9.5\text{ mg}$. For $200\text{ cm}^3$, the mass is calculated as $(200 / 1000) \times 9.5 = 0.2 \times 9.5 = 1.9\text{ mg}$.

(b) $\mathrm{PO}_4^{3-}$ AND $\mathrm{NH}_4^+$ OR $\mathrm{PO}_4^{3-}$ AND $\mathrm{NO}_3^-$

NPK fertilisers contain Nitrogen (N), Phosphorus (P), and Potassium (K). Since potassium ions provide K, the other two ions must provide N (ammonium $\mathrm{NH}_4^+$ or nitrate $\mathrm{NO}_3^-$) and P (phosphate $\mathrm{PO}_4^{3-}$).

(c) nitrate AND phosphate

High concentrations of nitrate ($\mathrm{NO}_3^-$) and phosphate ($\mathrm{PO}_4^{3-}$) ions act as nutrients, causing eutrophication. This leads to excessive growth of algae, whose subsequent decay by bacteria depletes the water of dissolved oxygen, damaging aquatic life.

(d) distillation

The apparatus shown involves boiling the river water and then condensing the steam. This is the process of simple distillation, used to separate a pure liquid (water) from dissolved solid impurities.