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[化學 Chem] Questions on Qualitative Analysis

Questions on Qualitative Analysis

Question 1:

In what circumstances do ions interfere each other?

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Solution to Question 1

When Zn[2+] ions and NH4[+] ions are present in a solution, Zn(OH)2 won't be precipitated in the presence of excess NH3, so if you conclude that Zn[2+] ions do not exist in the mixture, you are cooked.

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Question 2:

What should not be mixed with H2SO4(l)?

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Solution to Question 2

1. Group I Metals

2Na + H2SO4 ---> H2 + Na2SO4


2. MnO4[-]

H2SO4 + 2 KMnO4 ---> K2So4 + Mn2O7 + H2O
2Mn2O7 --> 4MnO2 + 3O2


3. ClOn[-]

4NaClO4 + 4H2SO4 --> 4NaHSO4 + 3O2 + 4ClO2 + 2H2O


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Question 3:

When using H2SO4(l) or concentrated HNO3, we shall do the reaction in fume cupboard. Why?

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Solution to Question 3

H2SO4(l) can yield toxic SO2(g) or SO3(g).

HNO3(aq) can yield toxic NO2(g).

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Question 4:

What reactions else shall we do in fume cupboard?

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Solution to Question 4

1. Heating solid

e.g. Cu(NO3)2 --> 2 NO2 + O2 + Cu

Toxic gas is generated.

2. Flame Test

e.g. Pb[2+] (g) is a toxic vapor.

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