(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?
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