The GATE Grind

GATE 2025 ES – Question 36

Environmental Chemistry · Principles of Water Chemistry · 2 marks · Multiple choice

A researcher adds $HgCl_2$ to water at pH 10 and calculates expected mercury concentration from the added mass. An instrument measuring only free $Hg^{2+}$ reports much less. Consider explanations 1 Mercury largely volatilized;2 Mercury formed aqueous complexes;3 Mercury precipitated. Which are appropriate?

  1. Only 1
  2. 1 and 2
  3. 1 and 3
  4. 2 and 3

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (D) 2 and 3

Explanation

The added mass estimates total mercury, while the instrument detects only free $Hg^{2+}$. At high pH, hydroxo complexes and sparingly soluble mercury phases can reduce the free-ion concentration without removing all mercury from the sample. Simple addition of HgCl₂ does not by itself establish major conversion to volatile elemental mercury. Thus **2 and 3,D**.

More Principles of Water Chemistry questions

All Principles of Water Chemistry questions →