How do we differentiate between Fe^ 3+ and Cr^ 3+ in group III?

JEE Main 2002 Chemistry question

How do we differentiate between and in group III?

(1)by taking excess of solution
(2)by increasing ion concentration
(3)by decreasing ion concentration
(4)both (b) and (c)Correct

Answer: (4)

Solution

Group III cations are precipitated as hydroxides by in the presence of . Increasing suppresses the ionisation of (common-ion effect), i.e. decreases ; at such a controlled low , the hydroxides precipitate selectively

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Question ID VK-037933