JEE Main 2023 · 1 February · evening · Single correct · +4 / −1
Which element is not present in Nessler's reagent?
- (a) Mercury
- (b) Iodine
- (c) Potassium
- (d) Oxygen
Answer and solution
Answer (d) Oxygen
Solution
Chemistry ma’am · 4 steps
Write the formula K₂[HgI₄] first; the answer is the one element missing from it.
- Make the reagent. Nessler's reagent is prepared by adding excess potassium iodide to mercury(II) chloride. The red HgI₂ that forms first dissolves in the extra iodide to give a complex.
HgI₂ + 2KI → K₂[HgI₄]
- Read the formula. The reagent compound is potassium tetraiodidomercurate(II), K₂[HgI₄]. Its elements are potassium, mercury and iodine. There is no oxygen in it.
- Role of KOH. The reagent is used in a KOH solution so that it can detect NH₄⁺ ions as a brown precipitate. The KOH only makes the medium alkaline. It is not part of the reagent compound.
- So. Oxygen is the element not present in Nessler's reagent, which is option (d).
The idea to keep. Nessler's reagent is K₂[HgI₄] in alkali: potassium, mercury and iodine only. With NH₄⁺ it gives a brown precipitate, the standard ammonium ion test.
Answer from the NTA final answer key. Solution by PrepWiser.