What was the hard NEET 2025 numerical on chlorine estimation by Carius method?
180 mg of an organic compound produces 143.5 mg of AgCl precipitate. Molar masses given are Ag = 108 g mol⁻¹ and Cl = 35.5 g mol⁻¹. The task is to find the percentage composition of chlorine. The official answer is 20.
Most aspirants know the basic stoichiometry yet lose marks on unit consistency. The official solution yields exactly 19.72 percent, which NEET accepts as 20 percent.
What is the official step-by-step solution for this Carius method question?
1 : 1 stoichiometry gives moles of Cl = 10^{-3} mol.
NEET takes this value as 20 percent. Every step keeps milli-gram and gram-per-mole consistent by treating 143.5 mg as 0.1435 g implicitly through the division.
What mistake leads students to select 25 percent instead of 20 percent?
Many replace the molar mass of AgCl with just the molar mass of Ag while calculating moles of precipitate. They use 108 g mol⁻¹ instead of 143.5 g mol⁻¹.
This produces:
Mass of Cl then becomes 1.33 × 10^{-3} × 35.5 ≈ 47.2 mg. The percentage arrives at (47.2 / 180) × 100 ≈ 26 percent, closest to the 25 percent option.
The precipitate AgCl must be treated with its full formula mass.
Why does unit handling make the Carius method question hard in NEET 2025?
This question carries a difficulty tier tagged as hard (3) with an expected solving time of 120 seconds. The key pitfall is keeping milli-gram and gram-mol consistent across every conversion.
It appeared in the NEET 2025 morning shift chemistry paper among 45 chemistry questions.
What are two similar estimation questions from principles of practical chemistry?
A 360 mg organic compound yields 564 mg of AgBr. Moles of AgBr equal 564 mg divided by 188 g mol⁻¹ gives 3 × 10^{-3} mol. Mass of Br is then 3 × 10^{-3} × 80 = 240 mg. Percentage of bromine equals (240 / 360) × 100 = 66.7 percent, which matches the 67 percent range in typical NEET options.
A 180 mg sample produces 233 mg of BaSO4. Moles of BaSO4 equal 233 mg / 233 g mol⁻¹ = 10^{-3} mol. Mass of S equals 10^{-3} × 32 = 32 mg. Percentage of sulphur equals (32 / 180) × 100 ≈ 17.8 percent, which falls in the 18 percent option range.
Both reuse the exact mole-to-mass chain. Solve them immediately after the main question to lock the method.
Search past NEET papers by chapter inside the past-paper archive to locate every worked solution of this type (100 free searches a month).
What is the revision formula card for halogen and sulphur estimations?
- %Cl = (35.5/143.5) × (mass AgCl/mass compound) × 100
- %Br = (80/188) × (mass AgBr/mass compound) × 100
- %S = (32/233) × (mass BaSO4/mass compound) × 100
Compare the molar-mass ratios side-by-side:
- Chlorine uses 35.5 over 143.5, roughly 0.247
- Bromine uses 80 over 188, roughly 0.426
- Sulphur uses 32 over 233, roughly 0.137
The numerators are always the atomic mass of the element. The denominators are always the molar mass of the precipitate. Multiply by the mass ratio and by 100.
How can you finish Carius method numericals in under 90 seconds?
Convert all masses to the same unit (mg or g) at the start. Write the 1 : 1 mole ratio explicitly before calculating mass of halogen. Perform the final division and multiply by 100 in one line. Recognise that 19.72 % is accepted as 20 % in NEET.
Repeat this checklist on every practice question. You can expect the 120-second expectation to drop once units and stoichiometry stay visible on paper. If a similar doubt appears during revision, photograph a doubt for an immediate step-by-step solution across Chemistry topics (20 free a month).
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Frequently asked questions
How do you solve the chlorine Carius method numerical in NEET 2025?
Calculate moles of AgCl by dividing 0.1435 g by 143.5 g mol⁻¹ to get 10^{-3} mol. With 1:1 stoichiometry, moles of Cl equal 10^{-3} mol. Mass of Cl is 35.5 mg, so percentage is (35.5/180)×100 = 19.72% which NEET accepts as 20%.
Why do many students choose 25% for the NEET chlorine percentage question?
Students often use the atomic mass of Ag (108 g mol⁻¹) instead of the molar mass of AgCl (143.5 g mol⁻¹) to find moles of precipitate. This overestimates moles of Cl at roughly 1.33×10^{-3}, giving mass of Cl near 47 mg and percentage around 26%, closest to the 25% option.
What is the revision formula for percentage of chlorine in Carius method?
%Cl = (35.5/143.5) × (mass AgCl/mass compound) × 100. The numerator is always the atomic mass of the element and the denominator is the molar mass of the precipitate. Use %Br = (80/188) × (mass AgBr/mass compound) × 100 and %S = (32/233) × (mass BaSO4/mass compound) × 100 similarly.
How to finish Carius method questions in under 90 seconds in NEET?
Convert masses to consistent units at the start and note the 1:1 mole ratio explicitly. Perform the full calculation in one line for the percentage. Remember that 19.72% is accepted as 20% by NEET. Practice the similar AgBr and BaSO4 questions immediately after to lock in the method.