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1991 WAEC Chemistry Practical (a) Name the apparatus used in the laboratory to (i) Convert vapour into liquid during...

Chemistry
WAEC 1991

(a) Name the apparatus used in the laboratory to (i) Convert vapour into liquid during distillation

(ii) determine the volumetric composition of water

(ii) produce an intermittent supply of any gas which can be evolved by the action of a liquid on a solid without heating.

(b) Give the reason for the following;

(i) the edge of the lid of a desiccator should be greased

(ii) a standard solution of sodium hydroxide is not prepared by weighing out accurately a given mass of the solid and making it up to the required volume.

iii) after the reduction of copper (II) oxide by a a stream of hydrogen gas passed overs the heated oxide in a combustion tube, it is necessary to continue passing the gas over the residue until the residue cools.

(c) An acid-base indicator turns orange in an acidic solution, green in a neutral solution and blue in an alkaline solution. The table shows the results obtained when a few drops of the indicator were added to samples of some liquids.

Liquid  Colour of the indicator in the liquid 

Glucose solution 

Rainwater 

Lime water 

Vinegar

Stomach powder solution 

Green

Pale orange 

Blue 

Orange 

Pale blue 

 

(i) Arrange the liquids in increasing order of their expected pH values.

(ii) Explain why the colour of the indicator in the sample of rainwater is pale orange whereas in pure water it is green.

(iii) A given crystalline solid is suspected to be either sodium chloride or ammonium tetraoxosulphate (VI). Describe how you would use the indicator to identify the solid.

(iv) State with reason whether or not phenolphthalein indicator can be used to distinguish between the samples of glucose solution and vinegar.

(v) What would be the colour of methyl orange indicator in the lime water sample? 

 

 

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Explanation

(a) Liebig's condenser.

(i) Hoffman voltameter.

(ii) Flat bottom flask with thistle funnel.

 

(b)(i) To make it air-tight and thus prevent atmospheric moisture

(ii) Due to sodium hydroxide being atmospheric

(iii) To prevent re-oxidation of the hot residue.

 

(c)(i) Vinegar, rain water, glucose solution, stomach solution, limewater, arranged in an increasing pH order.

(ii) Rainwater is acidic (contains dissolved gases e.g CO\(_2\)SO\(_2\), H\(_2\)S). Pure water is neutral.

(iii) 

Test  Observation Inference 

The solid sample + distilled water

+ few drops of indicator to give a solution 

solution turned green

Solution turned orange  

Solid is sodium 

Solid is sodium chlorides 

Solid is (NH\(_4\))\(_2\)SO\(_4\) since it undergoes hydrolysis to give an acid solution 

 

 

(iv) No since phenolpthalein is colourless in both solutions (V) yellow 


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WAEC May/June 2024 - Practice for Objective & Theory - From 1988 till date, download app now - 99995
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JAMB CBT 2024 - Candidates, Schools, Centres, Resellers - Get Ready!
WAEC May/June 2024 - Practice for Objective & Theory - From 1988 till date, download app now - 99995
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