A student was given a solid containing a mixture of nitrate salts. The sample completely dissolved in water, and upon addition of dilute HCl, 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?

a)

HCl addition --> H+ and Cl-, acidic condition + Cl-

from the list, there can't be Ag+, Pb+2, or Hg2+2, therefore, ignore those

b)

addition of H2S --> 2H+ and S-2; acidic conditions

This will precipitate group II ions --> Hg2+, Bi3+ , Cu2+ , Cd2+

therefore, ignore Bi+3, Cu+2 and Cd+2

c)

when we adjust pH to 8, this is basic, so there should be precipitation of:

Co2+, Ni2+, Fe3+, Mn2+, Cr3+, Al3+, Zn2+ as Sulfates

from the list:

Ca+2 can be present, since it will never precipitate --> only test flame will identify this

Ag+ can't be

Fe+3 --> could be

Cr+3 --> could be

Cu+2 --> can't be

Bi+3 --> can't be

Co+2 --> could be

K+ ---> could be, since it requires flame test