Question 3
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- State what would be observed when BaCl2 solution is added to a portion of a saturated Na2CO3 followed by dilute HCl in excess.
- A gas Q decolorized acidified KMnO4 solution. Suggest what Q could be.
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Name one substance used in the laboratory for drying each of the following substances:
- ammonia gas;
- carbon (IV) oxide.
- Give a reason why a given mass of sodium hydroxide pellets cannot be used to prepare a standard solution.
Observation
This question tested candidates’ knowledge of general skills and principles in chemistry practical. The performance of candidates in the question was below average.
In part (a), majority of the candidates could not state what would be observed when BaCl2 solution is added to a portion of saturated Na2CO3 followed by dilute HCl in excess.
In part (b), majority of the candidates lost mark to writing chemical formulae instead of IUPAC names.
In part (c), majority of the candidates mentioned that NaOH is deliquescent, but could not relate it to increase in mass.
The expected answers include:
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- White precipitate
- Effervescence/gas evolved
- Precipitate dissolves/disappears
- SO2 /H2S unsaturated hydrocarbon (accept any named unsaturated hydrocarbon)
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- Calcium oxide
- Calcium chloride/ concentrated tetraoxosulphate (VI) acid
- Sodium hydroxide (NaOH) is deliquescent, it absorbs water vapour (moisture)and carbon (IV) oxide from the air, and this would add to its mass