f4 chem問題

1.
a) and c) will produce a precipitate when mixed.

a) The white precipitate is calcium carbonate, CaCO3.

b) The white precipitate is barium sulphate, BaSO4.


2)
a) Add excess magnesium into dilute hydrochloric acid until all the acid is used (no bubbles are formed).
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Filter the mixture. Evaporate the solvent (water) of the filtrate to obtain a hot concentrated solution. Cool the solution slowly, and solid magnesium chloride is crystallized out. Filter the mixture, wash the residue with a small amount of cold ethanol, and then dry the residue with a filter paper. The dried residue obtained is magnesium chloride.

b) Add dilute silver nitrate solution to the potassium bromide solution, and a creamy precipitate of silver bromide is formed.
AgNO3(aq) + KBr(aq) → AgBr(s) + KNO3(aq)
In darkness, filter the mixture, wash the residue with a small amount of cold ethanol, and use a filter paper to dry the residue. The dries residue obtained is silver bromide.

c) Add excess dilute hydrochloric acid firstly in order to dissolve all calcium carbonate.
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
Then add dilute sodium sulphate solution to the resulting solution, and a white precipitate of calcium carbonate is formed.
CaCl2(aq) + Na2SO4(aq) → CaSO4(aq) + 2NaCl(aq)
Filter the mixture, wash the residue with a small amount of cold ethanol, and then dry the residue with a filter paper. The dried residue obtained is calcium sulphate.