All your burette readings (initials and final), as well as the size of your pipette, must be recorded but no account of experimental procedure is required. ...

Answer Details

(a) Adding barium chloride to saturated sodium trioxocarbonate(IV) gives a white precipitate of barium trioxocarbonate(IV), BaCO₃. On adding excess dilute HCl the precipitate dissolves with effervescence, and a colourless gas (carbon(IV) oxide) that turns lime water milky is evolved.
\(BaCl_2 + Na_2CO_3 \rightarrow BaCO_3 + 2NaCl\); \(BaCO_3 + 2HCl \rightarrow BaCl_2 + H_2O + CO_2\)

(ii) A gas that decolorizes acidified potassium tetraoxomanganate(VII) is a reducing gas; Q could be sulphur(IV) oxide (SO₂) (hydrogen sulphide would also fit).

(b) Drying agents:
(i) Ammonia gas: calcium oxide (quicklime, CaO).
(ii) Carbon(IV) oxide: concentrated tetraoxosulphate(VI) acid (or anhydrous calcium chloride).

(c) Sodium hydroxide pellets cannot be used to make a standard solution because NaOH is deliquescent and also absorbs carbon(IV) oxide from the air, so its measured mass is not accurate; it is therefore not a primary standard.