chem mc

Question

Which of the following reagents can be used to digstinguish between sodium sulphite solution and sodium sulphate solution?
(1)barium chloride solution
(2)acidified potassium permanganate solution
(3)potassium iodine solution

A.(1)only
B.(2)only
C.(1)(3)only
D.(2)(3)only

答案:B
想問一下
點解(1)barium chloride solution唔得o既?

Thx

Answer 1

For (1) barium chloride solution, it will react with sodium sulphite solution and sodium sulphate solution to give white insoluble barium sulphite and barium sulphate respectively.

Chemical equations:
BaCl2(aq) + Na2SO3(aq) → BaSO3(s) + 2NaCl(aq)
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)

As the two products have the similiar appearance, so barium chloride solution is not an appropriate chemical to distinguish between sodium sulphite solution and sodium sulphate solution. So, answer 1 is eliminated.

For answer 3 potassium iodine solution, the chemical equations go like these:
2KI(aq) + Na2SO3(aq) → K2SO3(aq) + 2NaI(aq)
2KI(aq) + Na2SO4(aq) → K2SO4(aq) + 2NaI(aq)
The reactants and the products are all colourless, so no observable change can be detected, and hence it cannot distinguish between the two solutions. So 3 is eliminated.

For 2 acidified potassium permanganate solution. It is an oxidizing agent. It will react with sodium sulphite solution by oxidizing it. However, it will not react with sodium sulphate solution as S in sulphate has the maximum oxidizing number +6, it cannot be further oxidized.
Chemical equations:
SO3 2-(aq) + MnO4-(aq) + 6H+(aq) + 3e- → SO4 2-(aq) + Mn2+(aq) + 3H2O(l)
The acidified potassium permanganate solution is decolourized when it is added to sodium sulphite solution while it has no colour change (remains purple) when added to sodium sulphate solution. So, only 2 is the answer.

So the answer is B.

Answer 2

Barium chloride react with sulphite and sulphate to form insoluble barium sulphite and barium sulphate. So cannot distinguish the sulphite and sulphate compound. If HCl is added afterward, the barium sulphite will be dissolved again while barium sulphate will not.

Please notified that the choose (1) is barium chloride solution , not barium chloride solution with HCl

Many students always choose the wrong answer with careless.

Answer 3

Both barium sulphite and barium sulphate are INSOLUBLE.

2006-10-16 17:51:28 補充：
p.s. All metal sulphates are soluble EXCEPT for CALCIUM, LEAD and BARIUM.All metal sulphites are insoluble EXCEPT for NH4 ion

2006-10-16 18:12:14 補充：
ohh wow Audrey Hepburn did a clear explanation! (: