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NVAEasyJEE 2025Qualitative Analysis (N, S, Halogens)

JEE Chemistry 2025 Question with Solution

In Carius method for estimation of halogens, 180 mg180 \, \text{mg} of an organic compound produced 143.5 mg143.5 \, \text{mg} of AgCl. The percentage composition of chlorine in the compound is _____ %. [Given: Molar mass in g mol−1\text{g mol}^{-1} of Ag = 108108, Cl = 35.535.5]

Answer

Correct answer:20

Step-by-step solution

Standard Method

Given: Mass of organic compound = 180 mg180 \, \text{mg} and mass of AgCl formed = 143.5 mg143.5 \, \text{mg}.

Find: Percentage composition of chlorine in the compound.

In Carius method, the moles of chlorine present are equal to the moles of AgCl formed.

nCl=nAgCl=143.5×10−3143.5=10−3n_{\text{Cl}} = n_{\text{AgCl}} = \frac{143.5 \times 10^{-3}}{143.5} = 10^{-3}

Mass of chlorine present:

mCl=10−3×35.5=35.5×10−3 gm_{\text{Cl}} = 10^{-3} \times 35.5 = 35.5 \times 10^{-3} \, \text{g}

Percentage composition of chlorine:

% Cl=10−3×35.5180×10−3×100=19.72%\% \, \text{Cl} = \frac{10^{-3} \times 35.5}{180 \times 10^{-3}} \times 100 = 19.72\%

Therefore, the percentage composition of chlorine is approximately 20%20\%.

Common mistakes

  • Using the mass of AgCl directly as the mass of chlorine is incorrect because AgCl contains both silver and chlorine. First convert AgCl to moles, then use the stoichiometric relation to find the mass of chlorine only.

  • Forgetting that the mole ratio of AgCl to Cl is 1:11:1 leads to an incorrect chlorine amount. Each mole of AgCl corresponds to exactly one mole of chlorine.

  • Mixing mg\text{mg} and g\text{g} carelessly can cause numerical errors. Convert consistently before substitution, or keep the same unit throughout the percentage calculation.

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