Question

2 . Esther was given a blueish unknown Group III solution. She started her analysis by buffering the solution to pH 9 with ammonia and ammonium chloride. After centrifuging the resulting cloudy mixture in her test tube she declared: "I already know what I have and don't have! I don't really have to go any further! But I will go through the motions anyway. She wrote down the colors of the ppt and solution in the tt and then added thioacetamide and warmed the mixture. A ppt that looked black resulted. When she treated the ppt with HCl solution black ppt remained and the supernatant liquid was bluish. After centrifuging and separating the mixture, she made the solution basic with ammonia, added hydrogen peroxide, and heated it in a water bath. The solution turned yellow but no ppt formed. "OK, she said. I could go further. But it is not really necessary." Since only three ions are possible, and they have such distinctive properties I am sure of what I have and don't a) What are the initial observations and possible conclusions that led her to decide that she knew what cations she had, or did not have, in her sample? 0 b) What conclusion was she able to draw after treating her ppt with HCI? c) What was it about the observation after the ammonia/peroxide treatment that convinced her of her earlier hunches?

Answer 1

a) Group III cations-Al3+,Cr2+,(Fe2+,Fe3+),Ni2+,Co2+,Zn2+

group reagent added=NH4Cl+NH4OH(ammonia and ammonium Chloride) to get pH=9.0 ,so that the cations precipitate as hydroxides.

Al3+ +3OH- <--->Al(OH)3(white ppt)

Cr3+ +3OH- <-----> Cr(OH)3( greyish green ppt)

Fe3+ +3OH-<--->Fe(OH)3 (rust or brown ppt)

cloudy solution -due to Al(OH)3(white ppt) -Al3+ present

In solution remains,

Ni2+ +6NH3 <--->[Ni(NH3)6]2+ (blue solution)

Co2+ +6NH3 <--->[Co(NH3)6]2+ (pink solution)

Zn2+ +4NH3 <--->[Zn(NH3)4]2+ (colorless)

thioacetamide test for Ni2+

Ni2+ +CH3CSNH2 +H2O --->NiS+CH3CONH2+2H+

NiS or nickel sulfide formed ,black ppt -Ni2+ present

NiS+2HCl-->NiCl2 +H2S (NiCl2 in solution ,color-blue)

Cr(OH)3(s) +3H2O2 +4OH- (basic medium)--->2CrO42- (yellow solution)+8H2O

Cr3+ present

b)

NiS or nickel sulfide formed ,black ppt -Ni2+ present

After adding HCl,blue solution of Ni2+ formed.

NiS+2HCl-->NiCl2 +H2S (NiCl2 in solution ,color-blue)

c)

Cr(OH)3(s) +3H2O2 +4OH- (basic medium)--->2CrO42- (yellow solution)+8H2O

Cr3+ present

Cr(OH)3(s) formed in basic medium (NH3 aq) ,was oxidized by H2O2 to give CrO42- chromate (yellow solution)