qualitative analysis

Question 1

detection of C and H

Answer 1

Rxns:

1. C+ 2CuO on heating gives CO2+2Cu
2. 2H+CuO on heating gives H2O+ Cu
3. CO2 turns Ca(OH)2 milky forming CaCO3
4. water in addition to CuSO4 turns anhydrous CuSO4 blue

Question 2

Lassaigne’s Test or sodium extract

Answer 2

Na+ C+ N on heating gives NaCN
2Na+ S on heating gives Na2S
Na+ X on heating gives NaX

Question 3

detection of N

Answer 3

1. NaCN + Fe2SO4 gives [Fe(CN)6]4- + 4Fe3+ + xH2O gives Fe4[Fe(CN)6]3.xH2O i.e prussian blue
2. if S is also present with N a red color in place of prussian blue appears due to formation of Fe(SCN)2+
3. hydrazine [N2H4] does not gives lassaigne’s test due to absence of C
4. it is not shown by diazonium salts also

Question 4

detection of S

Answer 4

1. S2- + Pb2+ gives PbS which is a black ppt

2. S2- +[Fe(CN)5NO]2- [sodium nitroprusside] gives [Fe(CN)5NOS]4- which is a white ppt

Question 5

detection of halogens

[BEILSTEIN Rxn is also for halogens however it’s not a confimatory test]

Answer 5

X-+ Ag+ gives AgX where -X= Cl, Br, I
AgCl is white ppt
AgBr is dull yellow ppt
AgI is bright yellow ppt

Question 6

detection of phosphorous

Answer 6

Na3PO4 +3HNO3 gives H3PO4 +3NaNO3

H3PO4+12(NH4)2MoO4 +21 HNO3 gives (NH4)3PO4.12MoO3 ammonium phospho molybdate [yellow ppt] +21 NH4NO3 +12 H2O