Which of the following gases is used for separating metal ions into respective groups in qualitative inorganic analysis?

Options

A. `H_(2)S`
B. `SO_(2)`
C. `SO_(3)`
D. both (2) and (3)

Answer

1

Explanation

Hydrogen sulphide is used to an qualitative and analysis for precipitaing the sulhides of many metals,in practice the preciptation is done under controlled conditions so that two groups of sulphide can be distinguised (Analysis of groups II and IV which are in no way related to the groups `2` and `4` of the periodic classification).
Gropus`II`-the sulphides of mercury`(II)`,lead ,bismuth ,coppr `(II)`, cadmium , aresenic,antimony and tin.
Group `(IV)-`the sulphdes of nickel ,cobalt,maganese, and zinc.
The syphides of Group II have smaller solubility products than those of Group IVi.e,they are less soluble in water, in order to ensure that the Group `II` sulphides precipitate but the Groups `IV` sulphides do not, the sulphide ion concentration must be decreases to an even smaller value than it already is in a saturated solution of hydrogen sulphide. This is acheived be carrying out the precipitation in the presence of acid (such as `HCl`),the additional as`H_(3)O(aq.)` ions shifting the two equilibria (shown below)to the left and thus reducting sulphide ion concentration in aqueous solution:
`H_(2)S(g)+H_(2)O(l) hArr H_(3)O^(+)(aq.)+HS^(-) (aq.)`
`HS^(-)(aq.)+H_(2)O(l) hArr H_(3)O^(+)(aq.)+S^(2-) (aq.)`
Both equilibria shifted to the left by the extra `H_(3)O^(+)` ions, For example ,both `CuS` and `NiS` precipitate when `H_(2)S` is passed through molar solution of `Cu^(2+)` and `Ni^(2+)` salt,but only `CuS` precipitates when the solution is made `0.3` molar with respect to hydrochloric acid.
To ensure the convicted precipitation of the Group `IV` sulphides,the solutions ,the solution is made alkalne(usually by adding aqueous ammonia solution in the presence of ammonium chloride ).The presence of he `OH^(-)` ensures a great sulphide ion concentration than that present in a netural solution of `H_(2)S` because the `OH^(-)` ions combined with `H_(3)O^(+)` ions, formed when the `H_(2)S` ionises, to form water, and more hydrogen sulphide ionises as a result.
`H_(2)S(g)+H_(2)O(l) hArr H_(3)O^(+) (aq.)+HS^(-) (aq.)`