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Question

What is the action of the Nessler's reagent on ammonia?


A

Brown ppt.

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B

Blue ppt.

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C

Black ppt.

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D

Green ppt.

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Solution

The correct option is A

Brown ppt.


Explanation for correct option

(A) Brown ppt.

Nessler's reagent

  • For the qualitative test of ammonia, Nessler's reagent is used.
  • Nessler's reagent is potassium tetraiodomercurate (II) K2HgI4.
  • It is an inorganic compound made up of potassium cations and the anion tetraiodomercurate (II).

Test for ammonia

  • The color of the complex generated will be determined by the color of the precipitate formed after the reaction of Nessler's reagent with ammonia NH3.
  • Potassium hydroxide KOH is added to make the solution of ammonia make slightly alkaline.
  • Potassium tetraiodomercurate(II) generates dirty brown precipitate when passed through an ammonia solution.
  • The reaction is as follows:

2K2HgI4(Potassiumtetraiodomercurate(II))+3KOHPotassiumHydroxide+NH3AmmoniaOHg2.NH2IIodideofMillon'sbaseBrownppt.+7KIPotassiumIodide+2H2OWater

Explanation for incorrect options

  • The brown ppt. is formed when Nessler's reagent reacts with ammonia.
  • Hence, all other options (B), (C) and (D) are incorrect.

Therefore the correct option is (A) Brown ppt​.


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