Question

Practical Q3(a)(i) State what would be observed when BaCl2 solution is added to a portion of a saturated Na2CO3 followed by dilute HCl in excess.
(ii) A gas Q decolorized acidified KMnO4 solution. Suggest what Q could be. [4 marks]
(b) Name one substance used in the laboratory for drying each of the following substances: (i) ammonia gas; (ii) carbon(IV) oxide [2 marks]
(c) Give a reason why a given mass of sodium hydroxide pellets cannot be used to prepare a standard solution. [2 marks]

Accepted Answer

(a)(i) A white precipitate (BaCO3) is formed with BaCl2; on adding excess dilute HCl, the white precipitate dissolves with the evolution of a colourless, odourless gas that turns lime water milky (CO2), i.e. Ba2++CO3²⁻→BaCO3, then BaCO3+2HCl→BaCl2+H2O+CO2.
(ii) Q is likely sulphur(IV) oxide, SO2 (a reducing gas that decolorizes acidified KMnO4 by reducing it).
(b)(i) Concentrated tetraoxosulphate(VI) acid is normally used to dry hydrogen chloride gas; ammonia gas itself is dried using calcium oxide (quicklime), since it is basic and would react with acidic drying agents.
(ii) Carbon(IV) oxide is dried using concentrated H2SO4.
(c) Sodium hydroxide pellets are deliquescent — they absorb moisture from the atmosphere, so their mass keeps changing/varying, meaning an accurate, fixed concentration (a true standard solution) cannot be obtained from them.

WAEC Chemistry 2019 Theory — Question 28

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