Question

An unknown solution may contain some of the cations listed below. One student preformed a series of experiments to determine what ions are in the solution.

Based upon the experiments performed by the students, the results are as follows:

a) On addition of 6 M HCl, a white precipitate results. The white precipitate is then mixed with NH3 where it dissolves.

b) On the addition of 1 ml (20 drops) of 1 M thioacetamide (CH3CSNH2), the test tube was then heated in a boiling water bath for 5 mins. Removed from heat to cool, then centrifuged the contents, remove the supernatant (S1) and keep the precipitate. The precipitate was then rinse in water. 2ml of distilled water was then added to the precipitate then followed by 15 drops of 6M potassium hydroxide (KOH). 1 drop of 1 M thioacetamide (CH3CSNH2) was then added to the precipitate and heated in a water bath for 3 mins. The contents were then centrifuged and the supernatant (S2). Distilled water was then added to the precipitate followed by 0.75 ml of 6 M NH3. The precipitate dissolves giving a deep blue color.

c) The supernatant (that is bolded in b (S2) ), 6 M HCl was then added until the solution turns acidic. No reaction occurred.

d) The supernatant (that is bolded in b (S1)), 0.75 ml of 5M ammonium chloride (NH4Cl), 6M NH3 was added until the solution turned basic and a precipitate was formed. 1 ml (20 drops) of 1 M thioacetamide (CH3CSNH2), was then added to the precipitate mixed an then heated in a water bath for 5 mins. The solution was then cooled, centrifuged and the supernatant (S3) removed. The precipitate was then washed three times with distilled water. 3ml (60 drops) of sulfate/hydrogen sulfate (SO42-/HSO4-) buffer was then added and all of the precipitate dissolved. 6M NaOH was then added to the solution and a reddish-brown precipitate was observed. The contents were then centrifuged and the supernatant (S4) was removed. 2 ml of distilled water was then added to the reddish-brown precipitate, followed by 1 ml (20 drops) of 6 M HCl, then 3 drps of 1M potassium thiocyanate (KSCN) and a dark red solution was then observed.

e) The supernatant (that is bolded in d (S4)), 6M of HCl was then added, followed by 3M NH3. The solution was then warmed in a water bath. No precipitate was observed. Classify the following ions as present, absent, or undetermined from the observations made above. (input the whole word)

Silver

Aluminium

Barium

Calcium

Zinc

Iron (Fe3+)

Copper

Antimony (Sb3+)

Nickel (Ni2+)

Answer 1

Qualittative analysis of metal cations

a) with 6M HCl white precipitate formed which dissolves upon addition of excess NH3 : Silver (Ag+) confirmed present.

b) With thioacetamide gave precipitate which dissolved upon addition of NH3 in basic condition : Confirms copper (Cu2+) present.

c) Absent Aluminium,Zinc

d) Formation of reddish brown precipitate confirms Ni2+ present.

Formation of a dark red solution upon addition of KSCN confirms Fe3+ is present.

e) absent barium, calcium, antimony