A student mistakenly added concentrated (12M) HCl to a sampleof unknown, Instead
ID: 685298 • Letter: A
Question
A student mistakenly added concentrated (12M) HCl to a sampleof unknown, Instead of adding 6M HCl, as directed 1) At first a large amount of light-colored precipitationformed, but some dissolved on further addition of concentratedHCl. Write an equation(s) for the reaction(s) that might haveoccured. A student mistakenly added concentrated (12M) HCl to a sampleof unknown, Instead of adding 6M HCl, as directed 1) At first a large amount of light-colored precipitationformed, but some dissolved on further addition of concentratedHCl. Write an equation(s) for the reaction(s) that might haveoccured.Explanation / Answer
originally all three ppt: Ag+ (aq) & Cl- (aq) --> AgCl (s) (Hg+)2 (aq) & 2 Cl- (aq) --> Hg2C2l (s) Pb+2 (aq) & 2Cl- (aq) --> PbCl2 (s) but the AgCl precipitate disappears into solution, forming the soluble complex ion [Ag+1 (Cl-1)2 ]^-1 when the large excess of Chloride ions was added by the concentrated HCl: AgCl (s) & Cl-1 (aq) --> [Ag (Cl)2 ]^-1 (aq) so the Ag+ was lost into the supernate & likely discarded ======================================... if however some AgCl had remained, it would disappear into solution when the ammonia was added: AgCl (s) & 2 NH3 (aq) ---> [Ag+ (NH3)2] ^+1 a soluble complex ion if the preson quit upon seeing the Hg2Cl2 turn black, they would not even know that some AgCl had made it this far.... they need to acidify, with HNO3, that supernate above the black mercury ppt , to destroy the [Ag+ (NH3)2] ^+1 & allow AgCl (s) to return.... if Ag+ was in the original unknown, & no AgCl returned upon acidifying the ammonia solution with HNO3, it means that the high conentration of Cl-1 from the con HCl put all of the AgCl into solution at the start as the [Ag (Cl)2 ]^-1 (aq) & likely discarded ======================================... "The precipitate was separated from the supernatant liquid in (1) and was found to be insoluble in hot water" at the temperature of the hot water, the precipiate PbCl2 dissociates into solution, it is the most unstable chloride precipitate of the the entire group: PbCl2 (s) --> in hot water --> Pb+2 (aq) & 2 Cl- (aq) we are supposed to wash the ppt with ice cold water, instead so the PbCl2, went off into the solution, & was likely discarded ===================================== so that the only ppt that would still remain would be the Hg2Cl2 which does a redox in the presence of ammonia: one Hg+1 is oxidized to Hg+2 & forms whitish ppt of Hg(NH2)Cl & one Hg+1 is reduced to Hg metal, which appears black the mix of the two ppts appears grey to black
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