Question

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?

NaNO3, Cr(NO3)3, Bi(NO3)3, Mn(NO3)3, Sn(NO3)3, Cd(NO3)2, Zn(NO3)2, Ca(NO3)2

Answer 1

(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 -----------> 2 H⁺ 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