Qualitative Analysis Of Anions

Question 1

What is meant by qualitative analysis?

Answer 1

It is a way of finding out which chemical elements or ions are present in a sample

Question 2

What are the observations a positive test gives?

Answer 2

The formation or disappearance of:

1) color

2) precipitate

3) gas

4) odor

5) heat

Question 3

How do you test for chloride ions?

Answer 3

Add HNO3 to the unknown solution to make it acidic, then add AgNO3, if a white ppt of AgCl was formed then its a positive test of chloride ion (AgNO3 + NaCl = AgCl + NaNO3)

Question 4

How do you test a solution for the presence of sulfate ion (SO4)?

Answer 4

Add HCl to the unknown to make it acidic, then add BaCl2, the formation of BaSO4 white ppt is a positive test for sulfate ions (BaCl2 + NaSO4 = BaSO4 + 2NaCl)

Question 5

How can you detect the presence of nitrate ions (NO3)?

Answer 5

Add H2SO4 to the unknown, making it acidic, then add 10 drops of saturated iron (ii) sulfate (FeSO4), the formation of brown ring of [FeNO]2+ Indicates the presence of nitrate ions

Question 6

How do you test for the presence of carbonate ions (CO3)?

Answer 6

Add HCl to the unknown if CO2 bubble was present then its a positive test for carbonate ions, (Na2CO3 + 2HCl = 2NaCl + H2O + CO2)

Question 7

How can you detect the presence of phosphate ions(PO4)?

Answer 7

Add about 10 drops of 6 M HNO3 and 5 drops of 1 M (NH4)2MoO4 to the centrifuge tube, heat the mixture in a boiling water bath for several minutes, if a yellow ppt. of (NH4)3[PO4 (MoO3)12] formed then it is a positive test for phosphate ion, (Na3PO4 + 12(NH4)2 MoO4 + 24HNO3 = (NH4)3[PO4(MoO12)] + 3NaNO3 + 21NH4NO3 + 12H2O)

Question 8

Why are the solutions acidified?

Answer 8

To remove the carbonate and phosphate ions as they can react with most other reagents to give wrong observations, but the acid should not contains the ions that we want to test