An unknown solid is entirely soluble in water. On addition of dilute HC1, a prec

Question

An unknown solid is entirely soluble in water. On addition of dilute HC1, a precipitate forms. After the precipitate is filtered off, the pH is adjusted to about 1 and H_2S is bubbled in; a precipitate again forms. After filtering off this precipitate, the pH is adjusted to 8 and H_2S is again added; no precipitate forms. No precipitate forms upon addition of (NH_4)_2 HPO_4 The remaining solution shows a yellow color in a flame test. Based on these observations, which of the following compounds might be present, which are definitely present, and which are definitely absent. Drag the appropriate items to their respective bins.

Explanation / Answer

The unknown solid is completely soluble in water. HgO is sparingly soluble in water and CdS is completely insoluble in water; therefore, the fact that the solid is completely soluble in water rules out the presence of CdS. Since HgO is sparingly soluble In water, we will keep it under consideration and say that HgO might be present.

The aqueous solution of the solid gives a precipitate with dilute HCl. This confirms the presence of Pb(NO3)2 which reacts with dilute HCl to give a white precipitate of PbCl2 as per the below equation:

Pb(NO3)2 (aq) + 2 HCl (aq) ------> PbCl2 (s) + 2 HNO3 (aq)

The precipitate is filtered off and H2S is passed through the residual filtrate in acidic medium (pH = 1). Formation of a precipitate definitely indicates the presence of Cd(NO3)2 which forms CdS as per the below equation:

Cd(NO3)2 (aq) + H2S (aq) --------> CdS (s) + 2 HNO3 (aq)

We have already said that HgO may be present and if present, HgO will give a precipitate with H2S as below:

HgO (aq) + H2S (aq) ------> HgS (s) + H2O

The precipitate was filtered off; the filtrate was made alkaline (pH = 8) and then H2S was bubbled through the filtrate. ZnS is precipitated in alkaline medium and the absence of a precipitate indicates the definite absence of ZnSO4.

(NH4)2HPO4 was added to the filtrate and no precipitate was formed. This is a very specific test for Mg and a negative result means that Mg salt is absent.

The remaining solution gives a yellow colour in the flame test. This indicates the definite presence of Na2SO4.

Might be present: HgO

Are definitely present: Pb(NO3)2, Cd(NO3)2, Na2SO4

Are definitely absent: CdS, ZnSO4

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