Chemistry WCH16 January 2021 Questions & Answers
Questions & Worked Solutions For Chemistry WCH16 January 2021
IAL Chemistry WCH16 January 2021 Questions - Complete Paper (pdf)
- A student carries out some tests on four aqueous solutions A, B, C and D.
One of the solutions is aqueous barium chloride, BaCl2(aq).
(a) The student is asked to add A to samples of B, C and D in separate test tubes, a small amount at a time, until there is no further change.
The container of solution A has a hazard label.
(i) Identify the hazard indicated by this label.
(ii) Describe how you would add small amounts of A until there is no further change. Name the apparatus you would use.
(b) (i) B is a blue solution. When A is added to B, the mixture first turns green and then gradually turns yellow.
Give the formula of the cation in B.
(ii) When A is added to C, vigorous effervescence occurs and the gas produced turns limewater cloudy.
Identify, by name or formula, the gas produced.
(iii) Suggest the identity, by name or formula, of the anion in C
(iv) Identify A by name or formula. Justify your answer.
(v) When A is added to D no change is seen.
A small amount of this mixture is added to B and a white precipitate forms.
Suggest what can be deduced about solutions B and D.
(vi) A concentrated solution of ammonia is added to B.
Initially a pale blue precipitate forms. When more ammonia is added, the precipitate dissolves forming a dark blue solution F.
Identify, by name or formula, the pale blue precipitate and the species responsible for the dark blue colour in F.
(vii) A solution of the sodium salt of EDTA, Na4EDTA, is added to a sample of solution F. The solution turns pale blue.
Write an equation for the reaction.
State symbols are not required. - Students were told to determine the concentration of a solution of potassium chlorate(V), KClO3 . Two methods were used: precipitation and titration.
- Azo dyes, such as Organol Brown, can be made from benzene, C6H6 , using the
reaction scheme shown.
Due to the toxicity of benzene, the first step is never carried out in a school laboratory.
(a) In the preparation of nitrobenzene, benzene is added slowly to a mixture of concentrated nitric and sulfuric acids.
The mixture is warmed at 55°C under reflux for 45 minutes. The reaction mixture is stirred continuously.
(i) State why a reflux condenser is needed when the mixture is warmed.
(ii) Draw a diagram of the apparatus used to warm under reflux in this experiment
(iii) Suggest why the reaction mixture is stirred continuously.
(b) The excess acid is removed from the reaction mixture. The layer containing nitrobenzene is separated and dried before being purified by distillation.
Identify a suitable drying agent.
(c) Nitrobenzene is then reduced to phenylamine, C6H5NH2. Phenylamine reacts with nitrous acid at a temperature between 0°C and 10°C to form a diazonium compound.
(i) Nitrous acid is formed in the reaction mixture using sodium nitrite and hydrochloric acid.
State why nitrous acid is generated in the reaction mixture instead of being obtained from a chemical supplier.
(ii) Explain why the temperature of the reaction between phenylamine and nitrous acid must be neither lower than 0°C nor higher than 10°C.
(d) Reaction of the diazonium compound with an alkaline solution of naphthalene-1-ol produces the solid azo dye, Organol Brown.
The solid is purified by recrystallisation.
(e) The melting temperature of the recrystallised Organol Brown is measured to check its purity.
State what you would observe if the sample was pure
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