JEE Main On 8 April 2017 Question 7
Question: A solution containing a group-IV cation gives a precipitate on passing, $ H _2S. $ . A solution of this precipitate in dil. HCl produces a white precipitate with NaOH solution and bluish-white precipitate with basic potassium ferrocyanide. The cation is : [JEE Online 08-04-2017]
Options:
A) $ M{n^{2+}} $
B) $ Z{n^{2+}} $
C) $ N{i^{2+}} $
D) $ C{o^{2+}} $
Show Answer
Answer:
Correct Answer: B
Solution:
- As we know that, the nickel ion, cobalt ion, and manganese ion do not give white precipitate when they react with hydrogen sulfide. Only zinc ion gives white residue or precipitate on the reaction of dihydrogen sulfide because ZnS and MnS are usually soluble in a cold solution of HCl, and NiS and CoS are generally insoluble. The reaction of zinc and dihydrogen sulfide is shown below.$Zn^{2+}+H_2S→ZnS↓+2H^+$ The zinc sulfide produces white precipitate of zinc hydroxide when the reaction with sodium hydroxide occurs. The reaction is shown below. $ZnS+2NaOH→Zn(OH)_2↓+Na_2S$ The zinc sulfide also forms bluish white precipitate with a basic solution of potassium ferrocyanide. The chemical reaction is shown below. $2ZnS+K4[Fe(CN) _6]→Zn_2[Fe(CN) _6]↓+2K_2S$ So, the zinc ion gives all the reaction and desirable precipitate which are mentioned in the above question. Therefore, the correct option for this question is D that is $Zn^{2+}$
