Qualitative Analysis - Result Question 53

####56. An aqueous blue coloured solution of a transition metal sulphate reacts with H2S in acidic medium to give a black precipitate A, which is insoluble in warm aqueous solution of KOH. The blue solution on treatment with KI in weakly acidic medium, turns yellow and produces a white precipitate B. Identify the transition metal ion. Write the chemical reactions involved in the formation of A and B.

(2000, 4M)

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Solution:

  1. The transition metal is Cu2+. The compound is CuSO45H2O. It dissolves in water to give blue coloured solution due to presence of Cu2+. On passing H2S(g) in acid medium of salt solution black precipitate of CuS is obtained which is not soluble in aqueous KOH solution.

CuSO4+H2SH+CuS black +H2SO4

On adding KI solution to aqueous solution of CuSO4, yellow solution of CuI2 is formed in the beginning which decompose into white ppt of CuI.

CuSO4+2KICuI2+K2SO4 2CuI22CuI White +I2



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