An unknown solution is to be tested for the anions or their derivatives, listed

Question

An unknown solution is to be tested for the anions or their derivatives, listed below. (Derivatives, for example, include: PO43-  or H3PO4). The following tests are performed on separate portions of the unknown.

a) When a test portion is acidified with HCl, and lead acetate is held in the mouth of the test tube, the paper turns black.

b) A test portion is acidificed with HCl, boiled and cooled; then Cl2 water and hexane are added. After the mixture is shaken, the hexane layer is a clear, light reddish brown color.

c) Addition of an excess of HNO3 produces a colorless gas that forms a white precipitate when bubbled through limewater.

On the basis of these tests considered as a whole, mark (below) a plus sign (+) for each ion or its derivative that is definitely present in the original solution, a minus sign (-) if the ion is definitely absent, or a question mark if there is no evidence that the ion is present or absent.

1) Cl-

2) Br-

3) I-

4) SO42-

5) SO32-

6) S2_

7) NO3-

8) CO32-

9) PO43-

Explanation / Answer

a) Na2S +Pb(CH3CO2H)2 yields lead sulphide, a black precipitate.

So the anion which answers this balck ppt is Sulphide ion S2-

b) Bromide Br- ion gives light brown color

c) CO2 when passed though lime water, its turns lime water milky due to formation of white CaCO3 pppt

So the anion must be CO32-

So only S2-, Br- , CO32- are present based on the tests

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