Qualitative Analysis - Result Question 5

####5. Upon treatment with ammoniacal H2S, the metal ion that precipitates as a sulphide is

(2013 Adv.)

(a) Fe (III)

(b) Al (III)

(c) Mg (II)

(d) Zn (II)

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Answer:

Correct Answer: 5. (d)

Solution:

  1. PLAN Ksp(ZnS) is very high and Zn2+ is precipitated as ZnS by high concentration of S2 formed when H2S is passed in ammoniacal solution.

H2SZn++S2(I)H++OHH2O(II)

Reaction (I) is favoured in forward side if H+is removed immediately by OH(NH4OH).

Zn2++S2ZnS White ppt 

Fe3+ and Al3+ are precipitated as hydroxide.



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