(a) Describe briefly a suitable procedure for preparing a pure sample of MgSO\(_4\) starting from MgO.
(b)(i) Mention two sources of water pollution.
(ii) Explain why the sample of air collected in the process of boiling water is richer in oxygen than atmospheric air
(iii) Mention one substance used as coagulant in water treatment plants.
(c)(i) State two physical porperties of chlorine.
CaOCl\(_{2(s)}\) + 2HCI\(_{(aq)}\) -> CaCl\(_{2(aq)}\) + H\(_2\)O\(_{(l)}\) + Cl\(_{2(g)}\)
Calculate the mass of bleaching powder that will produce 400cm\(^3\) of chlorine at 25\(^o\)C and a pressure of 1.20 x 10\(^5\) NM\(^{-2}\). [O = 16.0; Cl = 35.5; Ca = 40.0;1 mole of gas occupies 22.4 dm\(^3\) at s.t.p; standard pressure = 1.01 x 10\(^6\) Nm\(^{-2}\)]
(a) Preparing pure MgSO4 from MgO
Warm some dilute tetraoxosulphate(VI) acid in a beaker and add magnesium oxide (a base) a little at a time, stirring, until no more dissolves (excess MgO ensures all the acid is used up). Filter to remove the unreacted MgO. Evaporate the filtrate until saturated (to the point of crystallisation), allow to cool so that MgSO4·7H2O crystallises, then filter off and dry the crystals between filter papers.
\[ MgO + H_2SO_4 \to MgSO_4 + H_2O \]
(b) Water
(i) Two sources of water pollution: industrial effluents/waste; sewage or domestic waste (also agricultural run-off, oil spillage).
(ii) Oxygen is more soluble in water than nitrogen, so the dissolved air has a higher proportion of oxygen than atmospheric air; when this air is expelled by boiling, it is richer in oxygen (about 33% O2) than ordinary air (about 21% O2).
(iii) Coagulant: alum (aluminium sulphate).
(c) Chlorine
(i) Two physical properties: it is a greenish-yellow gas with a choking, pungent smell (also: denser than air, moderately soluble in water, poisonous).
(ii) \[ 2Fe + 3Cl_2 \to 2FeCl_3 \]
(iii) Chlorine is preferred to sulphur(IV) oxide because chlorine bleaches by oxidation, giving a permanent effect, whereas SO2 bleaches by reduction, which is temporary (the colour returns on exposure to air).
(d) Mass of bleaching powder
\[ CaOCl_2 + 2HCl \to CaCl_2 + H_2O + Cl_2 \]
Convert 400 cm3 of Cl2 at 25°C (298 K) and \(1.20 \times 10^5\ \text{N m}^{-2}\) to s.t.p. (273 K, \(1.01 \times 10^5\ \text{N m}^{-2}\)) using \( \dfrac{P_1V_1}{T_1} = \dfrac{P_2V_2}{T_2} \):
\[ V_2 = \frac{1.20\times10^5 \times 0.400 \times 273}{298 \times 1.01\times10^5} = 0.435\ \text{dm}^3 \]
Moles of Cl2 \( = \dfrac{0.435}{22.4} = 1.94 \times 10^{-2}\ \text{mol} \)
From the equation, 1 mol CaOCl2 gives 1 mol Cl2, so moles of CaOCl2 \( = 1.94 \times 10^{-2}\ \text{mol} \).
Molar mass of CaOCl2 \( = 40 + 16 + 71 = 127\ \text{g mol}^{-1} \)
Mass \( = 1.94 \times 10^{-2} \times 127 = \mathbf{2.47\ g} \)
(The stated "standard pressure = 1.01 × 106" is taken as 1.01 × 105 N m-2, the true standard pressure.)
