Mary Smith analyzed an unknown where the cations are all present as nitrates. (F
ID: 524709 • Letter: M
Question
Mary Smith analyzed an unknown where the cations are all present as nitrates. (For example, Ag+ if present is there as AgNO3, Pb2+ is present as Pb(NO3)2 etc.) Upon adding 6M HCl to her unknown, a precipitate appeared. To the supernatant S2, when aqueous NH3 was added, a precipitate was formed. When the precipitate was treated with NaOH and H2O2, the precipitate disappeared. Assume no mistakes were made in the experiment.
Which of the following statement or statements is/are most likely true of her unknown at this point of the analysis?
It is understood that the statements are still tentative as confirmation tests are yet to be conducted. For each statement below state whether it is most likely true or false. Give an explanation for each answer.
a) Ag+ must be in the unknown.
b) Pb2+must be in the unknown.
Explanation / Answer
Both a) and b) are true on the basis of given observation. Addition of 6M HCl to the unknown will result in the precipitation of of both Ag+ and Pb2+.
Ag+(aq) + Cl- (aq) AgCl (s)
Pb2+(aq) + 2Cl(aq) PbCl2(s)
Further, these two can be differentiated as PbCl2 will dissolve in hot water and AgCl is soluble in aqueous ammonia due to the formation of a complex ion.
PbCl2(s) + H2O(l) + heat Pb2+(aq) + 2Cl(aq)
AgCl(s) + 2NH3(aq) Ag(NH3)2(aq) + Cl(aq)
Now, it is mentioned that the suparnatant S2 is treated further with NH3, This supenatant contains the metals from other groups hence gives no significant indication or confirmation for the presence of Ag+ and Pb2+.