Question 3
(a) Give the reason for each of the following observations:
(i) sodium metal is stored under paraffin oil in the laboratory;
(ii) the edge of the lid of a desiccator is always greased;
(iii) 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.
[4 marks]
(b) (i) Sodium chloride is contaminated with methyl orange. Outline a suitable laboratory procedure to obtain pure crystals of sodium chloride from the solution.
(ii) Describe one chemical test to distinguish between hydrochloric acid and ethanoic acid.
[7 marks]
(c) (i) Name three pieces of apparatus that would be required to prepare a standard solution from a stock.
(ii) State what would be observed on adding dilute HCl to Na2SO3 (s).
[5 marks]
Observation
In part (a), the questions tested the knowledge of precautions that were taken in laboratory. Majority of the candidates could not give the reason why sodium metal is stored under paraffin oil. Also, they could not give the reason why the edge of lid of a desiccator is always greased.
In part (b), majority of the candidates could not outline a suitable laboratory procedure to obtain pure crystals of sodium chloride from the solution because of the contaminant.
In part (c), majority of the candidates could not state what would be observed when dilute HCl was added to Na2SO3(s).
The expected answers include:
(a) (i) To prevent the sodium from reacting with oxygen in the air.
(ii) To make it airtight / to prevent atmospheric moisture
(iii) Sodium hydroxide absorbs moisture / CO2 from air which increases the mass during weighing.
(b) (i) Add activated charcoal to remove the methyl orange. Heat / boil
for sometime and filter. Evaporate filtrate to dryness.
OR
1. Place the solution in a beaker and add few spatulaful of activated charcoal to remove methyl orange.
2. Boil/heat the mixture with stirring using a source of heat for sometime, and filter.
3. Place the filtrate in an evaporating dish, and evaporate to dryness.
(ii) Add AgNO3/Pb(NO3)2 solution to each sample separately. White precipitate indicates the presence of HCl. No visible reaction shows presence of ethanoic acid.
OR
Add ethanol and few drops of conc. H2SO4 to each sample separately and heat, a sweet smell of ester indicates the presence of ethanoic acid. No visible reaction shows the presence of HCl.
(c) (i)
- volumetric flask
- Pipette/burette
- funnel
- wash bottle containing distilled water
(ii)
- Effervescence/gas evolved
- Colour of the gas ………… colourless
- Odour of the gas ………….. … pungent/chocking/irritating
- Colour of the resulting solution - colourless
(ii) a colourless pungent / choking gas is evolved and a colourless solution is formed.