Question

A researcher added a certain amount of HgCl2 to water at pH 10. He calculated the expected concentration of mercury in the water. He asked his student to measure the concentration. His student used an instrument that can measure only the free metal, Hg2+. The student observed that the concentration measured by him was significantly less than the concentration calculated by the researcher. How can he explain the paradox to the researcher?

• A. Only Explanation 1 is correct.
• B. Explanations 1 and 2 are correct.
• C. Explanations 1 and 3 are correct.
• D. Explanations 2 and 3 are correct.

Hint:

Mercury can form complexes with chloride or hydroxide ions, especially at higher pH, which reduces the concentration of free \( {Hg}^{2+} \).

Answer 1

The Correct Option is D

Step 1: Understanding the behavior of HgCl2 at pH 10.
At pH 10, mercury in the form of HgCl2 may undergo different processes. The student’s measurement technique detects only the free mercury ion \( \text{Hg}^{2+} \), but other forms of mercury might be present, which are not detectable by the student’s instrument.

Step 2: Analyzing Explanations.
Explanation 1: The possibility of mercury changing phase from aqueous to gaseous is unlikely at standard conditions, as mercury vapor would need higher temperatures or specific conditions to volatilize. Hence, Explanation 1 is less likely to explain the discrepancy.
Explanation 2: Mercury at pH 10 can form various aqueous complexes, such as \( \text{HgCl}_3^- \) or \( \text{Hg(OH)}_2 \), which would reduce the concentration of free mercury \( \text{Hg}^{2+} \). This is a valid reason why the student observed a lower concentration.
Explanation 3: Mercury could have precipitated as mercury(II) hydroxide, \( \text{Hg(OH)}_2 \), especially at higher pH values, further reducing the free mercury concentration.

Thus, Explanations 2 and 3 correctly explain why the student observed a lower concentration of free mercury.