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

Question

An unknown solid is entirely soluble in water. On addition of dilute HCl, a precipitate forms. After the precipitate is filtered off, the pH is adjusted to about 1 and H2S is bubbled in; a precipitate again forms. After filtering off this precipitate, the pH is adjusted to 8 and H2S is again added; no precipitate forms. No precipitate forms upon addition of (NH4)2HPO4. 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.

CdS, Pb(NO3)2, HgO, ZnSO4, Cd(NO3)2, and Na2SO4

Explanation / Answer

The yellow flame necessarily indicates the pressence of Na.

So Na2SO4 is present.

The CdS is not soluble in water so CdS is not in the original compound

The addition of HCL creates chlorides with Pb

So Pb(NO3)2 is present in the compound.

We cannot be sure about HgO at this point because Cl- reacts with Hg2+2 not with ion Hg+2 which is the one that is present in HgO

PH = 1 and bubbling H2S

Under this conditions you should be able to identify ions from group II ( Hg+2 , Pb+2, Cu+2, Bi+3, Cd+2, Sn+2, As+3 and Sb+3.

So if there´s a precipitate you might have Cd(NO3)2 and HgO

The precipitates formed are:

CdS which is yellow

HgS which is black

PH = 8 and bubbling H2S

At this step you should be able to identify cations of group III (Fe+3, Cr+3, Al+3, Mn+2, Zn+2, Co+2, Ni+2)

With a slightly basic PH we should identify a white precipitate the ZnS. But since there´s not a precipitate we can say ZnSO4 is not present.

Summarizing the results we have:

Compound Existence

CdS No

Pb(NO3)2 Yes

HgO Migh be

ZnSO4 No

Cd(NO3)2 Might be

Na2SO4 Yes

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