(a) State how you would carry out the following procedures in the laboratory:
(i) Remove the sediment in sample of water;
(ii) Soften temporarily hard water without heating it,
(iii) Obtain pure water frdm muddy water;
(iv) Remove oxygen and moisture from a sample of air
(b)(i) What type of salts are alums?
(ii) State the function of alum in water treatment plants.
(iii) State and explain how rain water that hac passed through limestone deposits will react with soap solution.
(c)(i) Write an equation for the laboraton preparation of chlorine
(ii) List the products of the reaction of chorine with hot concentrated sodiuri hydroxide solution
(iii) What is observed when moist blue litmus paper comes in contact with chlorine?
(iv) Calculate the volume of chlorine at s.t.p. that would be required to react completely with 3.70g of dry slaked lime according to the following equation:
Ca(OH)\(_{2(s)}\) + Cl\(_{2(g)}\) --> CaOCl\(_{2}\). H\(_2\)O\(_{(s)}\) [H = 1, O = 16, Ca = 40, 1 mole of gas occupies 22.4 dm\(^3\) at s.t.p.]
(d)(i) State what is observed on warming ammonium trioxonitrate (V) with sodium hydroxide solution
(ii) Explain why ammonium trioxocarbonate (IV) leaves no residue on being heated.
(a) Laboratory procedures:
- (i) Remove sediment from water: by filtration (or decantation/sedimentation followed by filtration).
- (ii) Soften temporary hard water without heating: add a calculated amount of calcium hydroxide (slaked lime) - Clark's method - or add sodium trioxocarbonate(IV) (washing soda) to precipitate the Ca2+/Mg2+ ions.
- (iii) Obtain pure water from muddy water: by (simple) distillation (filter first, then distil).
- (iv) Remove oxygen and moisture from air: pass the air over heated copper to remove oxygen, then through a drying agent such as concentrated H2SO4 (or fused CaCl2) to remove moisture.
(b) (i) Alums are double salts. (ii) In water treatment, alum acts as a coagulant/flocculant: it causes the fine suspended clay/colloidal particles to clump together and settle out, clarifying the water. (iii) Rain water that has passed through limestone deposits dissolves calcium hydrogentrioxocarbonate(IV), becoming temporary hard water; with soap it gives little lather and forms an insoluble scum (because Ca2+ reacts with the soap) before it can lather.
(c) Chlorine
(i) Laboratory preparation: \(\text{MnO}_2 + 4\text{HCl} \to \text{MnCl}_2 + \text{Cl}_2 + 2\text{H}_2\text{O}\) (conc. HCl, warmed).
(ii) Products of chlorine with hot concentrated NaOH: sodium chloride, sodium trioxochlorate(V) (chlorate, NaClO3) and water: \(3\text{Cl}_2 + 6\text{NaOH} \to 5\text{NaCl} + \text{NaClO}_3 + 3\text{H}_2\text{O}\).
(iii) Moist blue litmus paper turns red and is then bleached (decolourised) by the chlorine.
(iv) Volume of Cl2 at s.t.p. to react with 3.70 g of slaked lime, Ca(OH)2 (molar mass = 74):
\[ \text{moles Ca(OH)}_2 = \frac{3.70}{74} = 0.05\ \text{mol} \]
The ratio Ca(OH)2 : Cl2 is 1 : 1, so moles Cl2 = 0.05 mol.
\[ \text{Volume} = 0.05 \times 22.4 = 1.12\ \text{dm}^3 \]
(d) (i) Warming ammonium trioxonitrate(V) with NaOH: a pungent gas (ammonia) is evolved that turns moist red litmus blue: \(\text{NH}_4\text{NO}_3 + \text{NaOH} \to \text{NaNO}_3 + \text{H}_2\text{O} + \text{NH}_3\). (ii) Ammonium trioxocarbonate(IV) leaves no residue on heating because it decomposes completely into gaseous products only (ammonia, carbon(IV) oxide and steam): \((\text{NH}_4)_2\text{CO}_3 \to 2\text{NH}_3 + \text{CO}_2 + \text{H}_2\text{O}\), all of which escape.
(a) Laboratory procedures:
- (i) Remove sediment from water: by filtration (or decantation/sedimentation followed by filtration).
- (ii) Soften temporary hard water without heating: add a calculated amount of calcium hydroxide (slaked lime) - Clark's method - or add sodium trioxocarbonate(IV) (washing soda) to precipitate the Ca2+/Mg2+ ions.
- (iii) Obtain pure water from muddy water: by (simple) distillation (filter first, then distil).
- (iv) Remove oxygen and moisture from air: pass the air over heated copper to remove oxygen, then through a drying agent such as concentrated H2SO4 (or fused CaCl2) to remove moisture.
(b) (i) Alums are double salts. (ii) In water treatment, alum acts as a coagulant/flocculant: it causes the fine suspended clay/colloidal particles to clump together and settle out, clarifying the water. (iii) Rain water that has passed through limestone deposits dissolves calcium hydrogentrioxocarbonate(IV), becoming temporary hard water; with soap it gives little lather and forms an insoluble scum (because Ca2+ reacts with the soap) before it can lather.
(c) Chlorine
(i) Laboratory preparation: \(\text{MnO}_2 + 4\text{HCl} \to \text{MnCl}_2 + \text{Cl}_2 + 2\text{H}_2\text{O}\) (conc. HCl, warmed).
(ii) Products of chlorine with hot concentrated NaOH: sodium chloride, sodium trioxochlorate(V) (chlorate, NaClO3) and water: \(3\text{Cl}_2 + 6\text{NaOH} \to 5\text{NaCl} + \text{NaClO}_3 + 3\text{H}_2\text{O}\).
(iii) Moist blue litmus paper turns red and is then bleached (decolourised) by the chlorine.
(iv) Volume of Cl2 at s.t.p. to react with 3.70 g of slaked lime, Ca(OH)2 (molar mass = 74):
\[ \text{moles Ca(OH)}_2 = \frac{3.70}{74} = 0.05\ \text{mol} \]
The ratio Ca(OH)2 : Cl2 is 1 : 1, so moles Cl2 = 0.05 mol.
\[ \text{Volume} = 0.05 \times 22.4 = 1.12\ \text{dm}^3 \]
(d) (i) Warming ammonium trioxonitrate(V) with NaOH: a pungent gas (ammonia) is evolved that turns moist red litmus blue: \(\text{NH}_4\text{NO}_3 + \text{NaOH} \to \text{NaNO}_3 + \text{H}_2\text{O} + \text{NH}_3\). (ii) Ammonium trioxocarbonate(IV) leaves no residue on heating because it decomposes completely into gaseous products only (ammonia, carbon(IV) oxide and steam): \((\text{NH}_4)_2\text{CO}_3 \to 2\text{NH}_3 + \text{CO}_2 + \text{H}_2\text{O}\), all of which escape.
