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The formula of the purple colour formed in Laissaigne’s test for sulphur using sodium nitroprusside is

1. NaFe[Fe(CN) 6 ]
2. Na[Cr(NH 3 ) 2 (NCS) 4 ]
3. Na 2 [Fe(CN) 5 (NO)]
4. Na 4 [Fe(CN) 5 (NOS)]

Correct answer: Na 4 [Fe(CN) 5 (NOS)]

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