Qualitative Analysis - Result Question 51

####54. A white substance A reacts with dilute H2SO4 to produce a colourless gas B and a colourless solution C. The reaction between B and acidified K2Cr2O7 solution produces a green solution and a slightly coloured precipitate D. The substance D burns in air to produce a gas E which reacts with B to yield D and a colourless liquid. Anhydrous copper sulphate is turned blue on addition of this colourless liquid. Addition of aqueous NH3 or NaOH to C produces first a precipitate, which dissolves in the excess of the respective reagent to produce a clear solution in each case. Identify A,B,C,D and E. Write the equations of the reactions involved. (2001, 10M)

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Solution:

  1. Since, the white substance A gives a colourless gas B with dil. H2SO4, such gas may be H2S. So, the substance A may be a metal sulphide (Na/K/Zn, etc.)

When H2S gas reacts with acidified K2Cr2O7, it gives green coloured solution of Cr2(SO4)3 alongwith slightly yellow ppt of D as sulphur.

K2Cr2O7+4H2SO4+3H2SK2SO4+Cr2(SO4)3 Green  +3S+7H2 D

S on burning in air gives SO2(E). Substance E on reaction with B(H2S) produces D(s) :

2H2SB+SO2E2H2O(l)+3SD

Anhydrous CuSO4 produces blue colour in water.

Solution C produces ppt first with NH3/NaOH which dissolve in excess NH3/NaOH. Hence, A must be ZnS.

ZnS+ dil. H2SO4AZnSO4(aq)+H2S(g)C ZnSO4+2NaOHCZn(OH)2 White +Na2SO4 Zn(OH)2+2NaOH[Zn(OH)4]2+2Na+



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