Viva Voce – Questions on Salt Analysis Practicals

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  • What is qualitative analysis?

The type of analysis that deals with the methods which are used to determine the constituents of a compound.

  • What is a radical?

A radical may be defined as an atom or group of atoms which carry charge & behaves like a single unit in chemical reactions.

  • What are preliminary tests?

The tests which are done for getting an indication of the radicals are called preliminary tests.

  • Name a few preliminary tests.

Physical examination of salt, dry heating, the action of diluted and concentrated sulphuric acid (H2SO4).

  • Which radicals are absent in a white salt?

Copper – Cu2+, Iron(II)-  Fe2+,  Iron(III) – Fe3+,  Cobalt – Co2+, and Nickel – Ni2+

  • Can sulphuric acid (H2SO4) be used for preparing the original solution?

No, because sulphuric acid (H2SO4) will produce sulphates of the cations and some sulphates like lead sulphate (PbSO4) are insoluble. So, we may not get the required clear original solution.

  • Explain how Aluminium (III) [Al3+] can be confirmed using cobalt nitrate.

Take a salt solution and add cobalt nitrate to it. Burn a filter paper which has been dipped into this solution. If you detect blue colour on black ash of filter paper, Aluminium (III) [Al3+] confirmed.

  • Why is ammonium chloride [NH4Cl] in III group? Can we add ammonium hydroxide (NH4OH) first?

NH4Cl is added to prevent precipitation of radicals other than III group as hydroxides by common ion effect. If we add NH4OH first, radical of group IV, V & VI will also precipitate and accordingly we cannot do it.

  • Why is it essential to boil off hydrogen sulphide [H2S] gas before precipitating III group?

If H2S isn’t boiled off group IV cations will also get precipitated.

  • How does the addition of ammonium hydroxide (NH4OH) help in precipitation of cations as sulphides?

Solubility product of Group IV sulphides is greater than those of group IV radicals. Thus, a higher concentration of S2- ions are required to precipitate Group IV radicals as sulphides. OH ions of ammonium hydroxide (NH4OH) with H+ ions of H2S, shift the equilibrium towards right or increasing the concentration of S2- ions.

  • Black precipitates in Group IV indicate which cations?

Presence of either Ni2+ or Co2+.

  • Why can sodium hydroxide [NaOH] not be used instead of ammonium chloride [NH4Cl] and ammonium hydroxide (NH4OH)?

NaOH is a very strong electrolyte and its ionization cannot be supressed. Moreover, it dissolves precipitates of Al3+, and Cr3+ as a result of formation of AlO2-, CrO42- complexes.

  • Can Barium Nitrate [Ba(NO3)2]solution be used instead of Barium Chloride [BaCl2]for testing sulphate ions [SO42-]?

Yes, since it is the Ba2+ ions which interact with SO42- and form the white precipitate of Barium sulphate [Ba SO4].

  • Sometimes a white precipitate is obtained in Group VI even in the absence of magnesium cation [Mg2+]. How?

This happens if the fifth group cations have not been precipitated there completely.

  • In the test for bromide ion [Br], why does only organic layer acquire brown colour?

Because Br2 formed is soluble in organic solvents.

  • Ammonium molybdate test applied for phosphate ions [PO43-] on a solution is positive but it doesn’t contain phosphate ions [PO43-] . How?

If the solution contains arsenate salt of arsenic, then this happens.

  • Why does Bromide not give chromyl chloride test?

Because chromyl chloride isn’t stable.

  • What is the chemical name of the yellow precipitate formed in Ammonium molybdate test of phosphate ion [PO43-] ?

Ammonium phosphomolybdate

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