Question

In the estimation of element X in an organic compound, 0.8 g of the compound containing X was heated with fuming HNO₃ and the cooled product was treated with barium chloride. The mass of barium sulphate precipitated was 1.2 g. What is the percentage of element X and what is the formula of the violet-coloured compound formed when Lassaigne's extract of the organic compound is treated with sodium nitroprusside?

• A. 30.24 % & Na₃[Fe(CN)₆NOS]
• B. 20.6 % & Na₄[Fe(CN)₆NOS]
• C. 40.27 % & Na₄[Fe(CN)₆NOS]
• D. 9.16 % & Na₃[Fe(CN)₆NO]

Hint:

In qualitative analysis, when barium chloride is added to the cooled reaction mixture after treatment with HNO₃, the formation of barium sulphate is an indication of the presence of sulphate ions (element X). The Lassaigne's test with sodium nitroprusside is useful for detecting sulfur in organic compounds.

Answer 1

The Correct Option is B

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: \[ \text{Molar mass of BaSO₄} = \text{Ba} + \text{S} + 4 \times \text{O} = 137 + 32 + 64 = 233 \, \text{g/mol} \] 2. Moles of Barium Sulphate The moles of barium sulphate precipitated is given by: \[ \text{moles of BaSO₄} = \frac{\text{mass of BaSO₄}}{\text{molar mass of BaSO₄}} = \frac{1.2 \, \text{g}}{233 \, \text{g/mol}} = 0.00515 \, \text{mol} \] 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: \[ \text{moles of X} = 0.00515 \, \text{mol} \] 4. Mass of Element X The mass of element X is: \[ \text{mass of X} = \text{moles of X} \times \text{atomic mass of X} \] From the stoichiometry and element identification, we deduce that X is sulfur, which has an atomic mass of 32 g/mol. Thus: \[ \text{mass of X} = 0.00515 \, \text{mol} \times 32 \, \text{g/mol} = 0.1648 \, \text{g} \] 5. Percentage of Element X The percentage of element X in the compound is: \[ \text{Percentage of X} = \frac{\text{mass of X}}{\text{total mass of compound}} \times 100 = \frac{0.1648}{0.8} \times 100 = 20.6 % \] 6. Formula of Violet-Coloured Compound The violet-coloured compound formed when Lassaigne's extract reacts with sodium nitroprusside is \(\text{Na}_4[\text{Fe(CN)}_6\text{NOS}]\), 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).