IIT-JEE 2007 · Paper 2 · Chemistry Q25

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ChemistrySingle correct+3 / -1Question 25 of 66

A solution of a metal ion when treated with KI gives a red precipitate which dissolves in excess KI to give a colourless solution. Moreover, the solution of metal ion on treatment with a solution of cobalt(II) thiocyanate gives rise to a deep blue crystalline precipitate. The metal ion is

\ce{Pb^{2+}}

\ce{Hg^{2+}}

\ce{Cu^{2+}}

\ce{Co^{2+}}

Answer and worked solution

Official answer: B

With KI. $\ce{Hg^{2+} + 2I- -> HgI2 v}$ (red), which dissolves in excess iodide: $\ce{HgI2 + 2I- -> [HgI4]^{2-}}$ (colourless).

With cobalt(II) thiocyanate. $\ce{Hg^{2+}}$ gives the deep blue crystalline $\ce{Co[Hg(SCN)4]}$ .

( $\ce{Pb^{2+}}$ gives yellow $\ce{PbI2}$ ; $\ce{Cu^{2+}}$ gives CuI and $\ce{I2}$ .)

(B) $\ce{Hg^{2+}}$

Topics:Cation analysis

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IIT-JEE 2007 · Paper 2, Chemistry question 25