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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3qc097

Select the correct statements about the preparation of the filtrate for group III.

AIron(II) is oxidised to iron(III) with concentrated $HNO X_{3}$ because $Fe (OH) X_{2}$ has a much larger $K_{s p}$ than $Fe (OH) X_{3}$
BBoiling off $H X_{2} S$ prevents precipitation of group IV sulphides together with group III in the ammoniacal medium
CAmmonium chloride limits $[OH X^{-}]$ so that $Mn (OH) X_{2}$ and $Mg (OH) X_{2}$ are not precipitated
DExcess $NH X_{4} OH$ is added so that $Al (OH) X_{3}$ redissolves as $[Al (NH X_{3}) X_{6}] X^{3 +}$

Answer: A, B, C

If

H X_{2} S

remained, ammonia would raise

[S X^{2 -}]

and precipitate sulphides of group IV prematurely. Fe(II) hydroxide is too soluble at the low

[OH X^{-}]

, and the common-ion effect of

NH X_{4} Cl

keeps Mn and Mg in solution. Al(OH)3 does not form an ammine complex, so the last statement is false.

Practice Qualitative Analysis: Cations More on Group III cations (Fe, Al, Cr)

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