Lassaigne S Test

The blue colour observed in Lassaigne's test for nitrogen is attributed to the formation of the complex compound called Prussian blue. This compound forms during the test when organic compounds containing nitrogen are converted into sodium cyanide (NaCN) through fusion with sodium. The cyanide ion then reacts with ferrous sulfate ( $%5Ctext%7BFeSO%7D_4$ ) to eventually form the blue-colored complex. The sequence of reactions is as follows:

Initially, the organic compound is fused with sodium to form sodium cyanide:
$%5Ctext%7BNa%7D%20%2B%20%5Ctext%7BC%7D%20%2B%20%5Ctext%7BN%7D%20%5Crightarrow%20%5Ctext%7BNaCN%7D$
Sodium cyanide reacts with ferrous sulfate to produce sodium hexacyanoferrate(II):
$6%5Ctext%7BNaCN%7D%20%2B%20%5Ctext%7BFeSO%7D_4%20%5Crightarrow%20%5Ctext%7BNa%7D_4%5B%5Ctext%7BFe(CN)%7D_6%5D%20%2B%20%5Ctext%7BNa%7D_2%5Ctext%7BSO%7D_4$
Upon addition of acid ( $%5Ctext%7BHCl%7D$ ) and further reaction with ferric sulfate ( $%5Ctext%7BFe%7D_2(%5Ctext%7BSO%7D_4)_3$ ), the Prussian blue complex is formed:
$3%5Ctext%7BNa%7D_4%5B%5Ctext%7BFe(CN)%7D_6%5D%20%2B%204%5Ctext%7BFe%7D_2(%5Ctext%7BSO%7D_4)_3%20%5Crightarrow%20%5Ctext%7BFe%7D_4%5B%5Ctext%7BFe(CN)%7D_6%5D_3%20%2B%206%5Ctext%7BNa%7D_2%5Ctext%7BSO%7D_4$

The correct answer is thus $%5Ctext%7BFe%7D_4%5B%5Ctext%7BFe(CN)%7D_6%5D_3$ , which is the chemical formula for Prussian blue, the blue coloration seen as a result of the test.

We are given the following information: - Mass of organic compound = 0.8 g - Mass of barium sulphate precipitated = 1.2 g - The chemical reaction involving HNO₃ and the barium chloride shows that the element X is in the form of sulphate. 1. Molar Mass of Barium Sulphate The molar mass of barium sulphate (BaSO₄) is calculated as:

%5Ctext%7BMolar%20mass%20of%20BaSO%E2%82%84%7D%20%3D%20%5Ctext%7BBa%7D%20%2B%20%5Ctext%7BS%7D%20%2B%204%20%5Ctimes%20%5Ctext%7BO%7D%20%3D%20137%20%2B%2032%20%2B%2064%20%3D%20233%20%5C%2C%20%5Ctext%7Bg%2Fmol%7D

2. Moles of Barium Sulphate The moles of barium sulphate precipitated is given by:

%5Ctext%7Bmoles%20of%20BaSO%E2%82%84%7D%20%3D%20%5Cfrac%7B%5Ctext%7Bmass%20of%20BaSO%E2%82%84%7D%7D%7B%5Ctext%7Bmolar%20mass%20of%20BaSO%E2%82%84%7D%7D%20%3D%20%5Cfrac%7B1.2%20%5C%2C%20%5Ctext%7Bg%7D%7D%7B233%20%5C%2C%20%5Ctext%7Bg%2Fmol%7D%7D%20%3D%200.00515%20%5C%2C%20%5Ctext%7Bmol%7D

3. Moles of Element X In the reaction, one mole of element X reacts to form one mole of barium sulphate. Hence, moles of X in the compound is equal to the moles of BaSO₄ formed:

%5Ctext%7Bmoles%20of%20X%7D%20%3D%200.00515%20%5C%2C%20%5Ctext%7Bmol%7D

4. Mass of Element X The mass of element X is:

%5Ctext%7Bmass%20of%20X%7D%20%3D%20%5Ctext%7Bmoles%20of%20X%7D%20%5Ctimes%20%5Ctext%7Batomic%20mass%20of%20X%7D

From the stoichiometry and element identification, we deduce that X is sulfur, which has an atomic mass of 32 g/mol. Thus:

%5Ctext%7Bmass%20of%20X%7D%20%3D%200.00515%20%5C%2C%20%5Ctext%7Bmol%7D%20%5Ctimes%2032%20%5C%2C%20%5Ctext%7Bg%2Fmol%7D%20%3D%200.1648%20%5C%2C%20%5Ctext%7Bg%7D

5. Percentage of Element X The percentage of element X in the compound is:

%5Ctext%7BPercentage%20of%20X%7D%20%3D%20%5Cfrac%7B%5Ctext%7Bmass%20of%20X%7D%7D%7B%5Ctext%7Btotal%20mass%20of%20compound%7D%7D%20%5Ctimes%20100%20%3D%20%5Cfrac%7B0.1648%7D%7B0.8%7D%20%5Ctimes%20100%20%3D%2020.6%20%25

6. Formula of Violet-Coloured Compound The violet-coloured compound formed when Lassaigne's extract reacts with sodium nitroprusside is

%5Ctext%7BNa%7D_4%5B%5Ctext%7BFe(CN)%7D_6%5Ctext%7BNOS%7D%5D

, which corresponds to the formula in option (B). Thus, the percentage of element X is 20.6% and the formula of the violet-coloured compound is Na₄[Fe(CN)₆NOS], corresponding to option (B).

High-Yield Trend

Chapter Questions
2 MCQs

Official Solution

Official Solution

Lassaigne S Test

High-Yield Trend

Chapter Questions 2 MCQs

Official Solution

Official Solution

Chapter Questions
2 MCQs