Qualitative Analysis - Problem 2

Description:

Which among the given statements is false?

A. Sodium reacts with the elements in an organic compound to form water soluble salts.

B. Lassaigne’s test for nitrogen on H₂N − C₆H₄ − SO₃H gives Prussian Blue colour.

C. Sodium cyanide gives positive Lassaigne’s test.

D. Lassaigne’s test for halogens uses HNO₃ as Na₂S and NaCN are decomposed by HNO₃ Solution:

Solution −

The correct option is: (B)

Explanation −

• Option (A): True

Sodium reacts with the elements in an organic compound to convert the covalently bound elements to ionic form and to water soluble salts.

• Option (B): False

Lassaigne’s test for nitrogen on H₂N − C₆H₄ − SO₃H gives blood red coloration and not Prussian blue due to the presence of both nitrogen and Sulphur giving SCN^- instead CN⁻ion.

• Option (C): True

• Option (D): True

In order to remove the interference from other elements such as nitrogen and Sulphur which exist as NaCN and Na₂S in the Lassaigne’s extract, conc.HNO₃ is added to remove these as HCN and H₂S gases.

NaCN + HNO₃ → NaNO₃ + HCN ↑

Na₂S + 2HNO₃ → 2NaNO₃ + H₂S ↑