A solution contains the sodium salt of an anion. The following tests are perform

Question

A solution contains the sodium salt of an anion. The following tests are performed to identify the anion. (a) The solution is treated with silver nitrate solution. A white precipitate forms. List all anions that could be present. (b) Addition of nitric acid to another sample of the SAME solution produced a colorless, odorless gas. On the basis of this test, what anions are present? (c) When another sample of the SAME solution was treated with magnesium chloride, there was a white precipitate. What single anion is present?

Explanation / Answer

1.a. The ions which precipitate with AgNO3 are Cl-, Br-, I-, CN-, SCN-, CO32-. These ions give white precipitate when react with AgNO3. Ag+ combines with these ions and preicpitate.

AgNO3 + NaX --------> AgX + NaNO3

AgX precipitated as white.

b. The colourless odourless gas can be produced on addition of HNO3 indicated the evolution of CO2 gas. The anions which can be present are HCO3- and CO32-.

Na2CO3 + HNO3 -----> NaNO3 + CO2 + H2O.

c. On treatment with MgCl2, the white precipitate is formed. HCO3- and CO32- gives white precipitate on reaction with MgCl2. But only CO32- can give AgNO3 test. So, anion present in salt is CO32- and the salt is Na2CO3.

MgCl2 + Na2CO3 -----------> MgCO3 + NaCl

MgCO3 settle as white precipitates.

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