# Quantitative Analysis of Organic Compounds

> Quantitative organic analysis for JEE Chemistry: C and H by combustion, Dumas and Kjeldahl nitrogen, Carius halogen and sulphur with worked sums.

- Canonical URL: https://prepelephant.com/topics/jee/chemistry/estimation-quantitative-analysis
- Exam / course: JEE · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Quantitative Analysis of Organic Compounds", PrepElephant, https://prepelephant.com/topics/jee/chemistry/estimation-quantitative-analysis

## Direct answer

Quantitative analysis pins down the exact percentage of each element in an organic compound, and every method ends in one gravimetric ratio. Carbon and hydrogen burn to CO2 and H2O, caught in potassium hydroxide and anhydrous calcium chloride respectively; nitrogen emerges either as N2 gas (Dumas) or as ammonia distilled into standard acid (Kjeldahl); halogens and sulphur go through the Carius tube as silver halide and barium sulphate. The arithmetic is always the same shape: mass of element inside the weighed product, divided by mass of compound, times one hundred — so Kjeldahl's 1.4 NV/w, Dumas' 28 V/22400 w, and the Carius factors are the only numbers to memorise.

## What you must remember

- **Carbon and hydrogen:** % C = 12/44 × mass of CO2 × 100/w; % H = 2/18 × mass of H2O × 100/w — the 12/44 and 2/18 factors do all the work.
- **Dumas:** % N = 28 × V × 100/(22400 × w), V in millilitres of N2 at STP collected over KOH (which absorbs CO2 and water vapour).
- **Kjeldahl:** % N = 1.4 × N × V / w, N the normality and V the millilitres of standard acid neutralised by the distilled ammonia.
- **Kjeldahl's blind spots:** nitro, azo and pyridine-type ring nitrogens never convert to ammonium sulphate — nitrobenzene must be measured by Dumas, a recurring one-liner.
- **Carius halogen:** % Cl = 35.5/143.5 × mass AgCl × 100/w (use 80/188 for AgBr, 127/235 for AgI); sulphur as BaSO4 with factor 32/233; phosphorus as Mg2P2O7 with 62/222.
- **Oxygen:** never measured directly — 100 minus the sum of all other percentages.

## Two clean calculations

Dumas first. A 0.30 g sample yields 50 mL of nitrogen at STP. Mass of nitrogen = 28 × 50/22400 = 0.0625 g, so % N = 0.0625 × 100/0.30 = 20.8%. If the compound also carried 6.67% hydrogen and 40% carbon from a combustion run, oxygen closes the account at 100 − 40 − 6.67 − 20.8 = 32.5%, and an empirical formula question can now begin. That hand-off — percentages to mole ratios — is the real destination of these numbers.

Carius second. A 0.20 g organic compound, heated with fuming nitric acid and silver nitrate in a sealed Carius tube, gives 0.147 g of AgCl. % Cl = 35.5/143.5 × 0.147 × 100/0.20 = 0.2474 × 0.147 × 500 = 18.2%. The factor 35.5/143.5 is simply the chlorine mass fraction of AgCl — nothing more exotic, which is why it can be reconstructed under exam pressure if forgotten.

## Where students slip

Unit slips decide marks here: volumes in Dumas must sit at STP (22,400 mL) — a room-temperature volume needs correction; the Kjeldahl formula uses normality (not molarity) of the acid and volume in millilitres against mass in grams; mixing systems shifts answers by factors of two or a thousand. The conceptual traps are Kjeldahl's applicability (candidates apply it to nitrobenzene out of habit) and the identity of the absorbing species in each train — KOH takes both CO2 and water vapour in the Dumas setup, while CaCl2 takes only water in the combustion train. JEE Main increasingly asks integer-percentage answers, so keep one decimal of discipline throughout; Advanced frames the follow-on, converting percentages to an empirical formula with an oxygen remainder — always check that the percentages close below 100 before dividing by atomic masses.

## Frequently asked questions

### Why does the Kjeldahl method fail for nitrobenzene?

Its nitrogen is bound in nitro (and azo, or pyridine-type ring) form, which the digestion with sulphuric acid does not convert to ammonium sulphate, so no ammonia is ever distilled.

### What absorbs what in the combustion estimation of C and H?

Water is absorbed first in anhydrous calcium chloride (or magnesium perchlorate), and carbon dioxide in potassium hydroxide — weighing each tube in turn gives the H and C masses.

### How is nitrogen measured in the Dumas method?

The compound burns with copper oxide in a CO2 stream, and liberated N2 collects over potassium hydroxide solution, which absorbs everything else; its STP volume feeds 28V/22400.

### What is the gravimetric factor for sulphur in the Carius method?

32/233 — the sulphur mass fraction of BaSO4, since every mole of sulphur in the compound yields one mole of barium sulphate.

### How is the oxygen percentage of a compound obtained?

By difference: 100 minus the sum of all measured percentages — direct oxygen estimation is not done in these classical methods.