COMEDK · Chemistry · 21. General Principles and Processes of Isolation of Metals

Sodium cyanide is added as a depressant in the froth floatation process when the ore contains a mixture of \(\mathrm{ZnS}\) and PbS. This is because

A \(\mathrm{Pb}(\mathrm{CN})_{2}\) gets precipitated without any effect on \(\mathrm{ZnS}\)
B ZnS forms soluble complex, while PbS forms froth.
C PbS forms soluble complex, while \(\mathrm{ZnS}\) forms froth.
D \(\mathrm{Zn}\left(\mathrm{CN}_{2}\right.\) gets precipitated without any effect on PbS

Verified Solution

Answer & Solution

Correct Answer

(B) ZnS forms soluble complex, while PbS forms froth.

Step-by-step Solution

Detailed explanation

\(\mathrm{NaCN}\) forms a soluble complex with \(\mathrm{ZnS}\) thus, it selectively prevents \(\mathrm{ZnS}\) from coming to the froth but allows \(\mathrm{PbS}\) to come with the froth.

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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