Analytical ChemistryHard
Question
A mixture of chlorides of copper, cadmium, chromium, iron and aluminium was dissolved in water. It was acidified with dilute HCl and then hydrogen sulphide gas was passed for sufficient time. It was filtered, boiled and a few drops of nitric acid were added, while boiling. To this solution ammonium chloride and ammonium hydroxide were added. To this excess of sodium hydroxide was added and then filtered. The filtrate shall give test for :
Options
A.sodium and iron
B.sodium, chromium and aluminium
C.aluminium and iron
D.sodium, iron, cadmium and aluminium
Solution
In first step with H2S gas in acidic medium Cd2+ and Cu2+ will precipitate as CdS and CuS respectively. Thus the filtrate will now contain chromium, iron and aluminium.
In second step with NH4Cl and NH4OH, Cr3+, Fe3+ and Al3+ will precipitate out as their hydroxides.
In third step on addition of excess of NaOH, Al(OH)3 and Cr(OH)3 precipitates will dissolve forming soluble complexes but Fe(OH)3 precipitate is unaffected. Hence the filtrate will contain Na[Al(OH)4] and Na[Cr(OH)4].
Al(OH)3 + OH- [Al(OH)4] - (colourless solution); Cr(OH)3 + OH- ⇋ [Cr(OH)4] - (green solution)
In second step with NH4Cl and NH4OH, Cr3+, Fe3+ and Al3+ will precipitate out as their hydroxides.
In third step on addition of excess of NaOH, Al(OH)3 and Cr(OH)3 precipitates will dissolve forming soluble complexes but Fe(OH)3 precipitate is unaffected. Hence the filtrate will contain Na[Al(OH)4] and Na[Cr(OH)4].
Al(OH)3 + OH- [Al(OH)4] - (colourless solution); Cr(OH)3 + OH- ⇋ [Cr(OH)4] - (green solution)
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