8/13/2016 11:55 PM A 87.9/100 8/12/2016 02:07 PM Gradebook Print calculator Peri

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8/13/2016 11:55 PM A 87.9/100 8/12/2016 02:07 PM Gradebook Print calculator Periodic Table Question 21 of 22 Map A Sapling Learning learning A student was given a solid containing a mixture of nitrate salts. The sample completely dissolved in water, and upon addition of dilute HCI, no precipitate formed. The pH was lowered to about 1 and H2S was bubbled through the solution. No precipitate formed. The pH was adjusted to 8 and H2S was again bubbled in. This time, a precipitate formed. Which compounds might have been present in the unknown? Ca(NO3)2 Pb (NO3)2 Cu (NO3)2 Zn (NO3)2 Cr(NO3)3 Fe (NO3): NaNO3 AgNO A Previous 3 Give Up & View Solution e Check Answer Next d Exit Hint

Explanation / Answer

the correct answer is Cr(NO3)3.

Group I (Ag+, Pb2+, Hg2+) cations produce insoluble chlorides so they can be precipitated withdilute HCl, while all other cations remain in solution.

Group II (Cu2+, Bi3+, Cd2+, Hg2+, As3+, Sb3+, Sn4+) cations produce very insoluble sulfides(Ksp values less than 10-30) so they can be precipitated by low amounts of sulfide ion; this can beachieved by adding an acidic solution of H2S.

Group III (Al3+, Cr3+, Fe3+, Zn2+, Ni2+, Co2+, Mn2+) cations produce slightly soluble sulfides(Ksp values more than 10-20) so they can be precipitated by relatively high amounts of sulfide ion;this can be achieved by adding a basic solution of H2S.

Group IV (Mg2+, Ca2+, Sr2+, Ba2+) cations, as well as all of the above groups, produceinsoluble carbonates so they can be precipitated by the addition of carbonate once the ions of thefirst three groups have been removed.

Group V (Na+, K+, NH4+) cations do not precipitate with any of the above reagents.

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