Question 5
- (i) Name three gases that should be prepared in a fume chamber.
(ii) Give a reason for your answer in 5(a)(i).
(iv) State one use of fused calcium chloride. [5 marks] - (i) Describe briefly the laboratory preparation of dry ammonia gas.
(ii) Explain briefly why ammonia cannot be dried with concentrated tetraoxosulphate (VI) acid. [5 marks] - (i) Name the gas produced during the action of warm dilute HCl on:
I. FeS;
II. a mixture of Fe and S.
(ii) Describe one chemical test to identify the gas produced in 5 (c)(i) II.
(iv) Give the reason why: I. CuSO4 solution is added to Zn and HCl during the preparation of hydrogen gas;
II. chlorine is used for water treatment. [6 marks] - On analysis, 1.0 g sample of a hydrocarbon was found to contain 0.923 g of carbon. If the vapour density of the hydrocarbon is 39.0:
(i) determine its molecular formula;
(ii) name the compound. [H = 1.0; C = 12.0]
Observation
Majority of the candidates avoided this question.
In part (a), majority of the candidates that answered this question named three gases that should be prepared in a fume chamber and correctly gave a reason for their answer.
In part (b), majority of the candidates could not describe the laboratory preparation of dry ammonia gas.
In part (c), few of the candidates named the gas produced during the action of warm HCl on FeS and a mixture of Fe and S.
In part (d), majority of the candidates could not determine the molecular formula of the hydrocarbon and the name of the compound.
The expected answers include:
-
(i) - sulphur (IV) oxide
- hydrogen chloride
- nitrogen (IV) oxide
- carbon (II) sulphide
- hydrogen sulphide
- chlorine
- ammonia
(ii) the gases are poisonous / toxic / can have effect on the health of an individual
(iii) - as a drying agent / dessicant
- for de-icing
- in dust control -
(i) Heat a mixture of an alkali ( e.g. NaOH, Ca(OH)2 / slaked lime, KOH)and an ammonium containing compound ( e.g. NH4Cl, NH4NO3) , (NH4)2 SO4.
The gas produced is dried over quicklime / CaO and collected by upward delivery / downward displacement of air.
(Do not accept industrial preparation of ammonia by Haber’s process)
(ii) It reacts with H2SO4 (to form (NH4)2 SO4 / a salt) -
(i) I Hydrogen Sulphide
II Hydrogen
(ii) Introduce a lighted splint into the gas, gas burns with a pop sound
(iii) I. CuSO4 acts as a catalyst
II. To kill germs / bacteria / microorganisms / germicide -
(ii) Benzene