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Lassaigne's test tells you whether an organic compound contains nitrogen. The next question is how much. Class 11 chemistry gives two methods, Dumas and Kjeldahl, and each comes with a formula that students tend to memorise without seeing where it comes from. This post builds both formulas, works one numerical for each, and lists the mistakes that cost marks.
Where is this in the syllabus?
Both methods are in section 8.10.2 of NCERT Class 11 chemistry, Unit 8, Organic Chemistry: Some Basic Principles and Techniques. The JEE (Main) 2026 syllabus lists "Quantitative analysis (basic principles only): estimation of carbon, hydrogen, nitrogen, halogens, sulphur and phosphorus" with numerical problems, and the NEET (UG) 2026 syllabus has the same line. The JEE (Advanced) 2026 brochure asks only for empirical and molecular formulae "by combustion method only", so Dumas and Kjeldahl numericals matter mainly for CBSE, JEE Main and NEET. Check the current bulletin for your exam.
How does the Dumas method work?
NCERT describes it like this. The compound is heated with copper(II) oxide in an atmosphere of carbon dioxide. Carbon becomes CO2, hydrogen becomes water, and nitrogen is released as N2. Any nitrogen oxides formed are reduced back to nitrogen over a heated copper gauze. The gas mixture is passed over potassium hydroxide solution, which absorbs the CO2, and the nitrogen collects in a graduated tube called a nitrometer.
Because the gas is collected over an aqueous solution, it is mixed with water vapour. So the pressure of nitrogen is:
p(N2) = atmospheric pressure − aqueous tension
Convert the volume to STP, then use NCERT's value that 22,400 mL of N2 at STP (273 K, 760 mm) weighs 28 g:
%N = 28 × V(STP) × 100 / (22400 × m)
Worked example 1: a Dumas numerical
These are sample numbers to show the method. A 0.250 g sample gives 40.0 mL of nitrogen at 290 K and 750 mm pressure. The aqueous tension at 290 K is given as 14 mm.
Pressure of dry nitrogen = 750 − 14 = 736 mm
Volume at STP = (736 × 40.0 × 273) / (760 × 290) = 36.47 mL
Mass of nitrogen = 28 × 36.47 / 22400 = 0.04558 g
%N = 0.04558 / 0.250 × 100 = 18.2%
How does the Kjeldahl method work?
The compound is heated with concentrated sulphuric acid (NCERT's diagram also shows copper sulphate in the flask), which converts its nitrogen to ammonium sulphate. Excess sodium hydroxide then releases ammonia, which is distilled into a known volume of standard sulphuric acid. The acid left over is titrated with standard alkali. The difference tells you how much acid the ammonia used. Johan Kjeldahl published the method in 1883.
NCERT's derivation uses the same molarity M for the acid and the alkali. If V mL of H2SO4 was taken and V1 mL of NaOH neutralised the excess, then:
Excess acid = V1/2 mL (each H2SO4 neutralises two NaOH)
Acid used by ammonia = V − V1/2 mL
Ammonia = 2(V − V1/2) mL of molarity M (each H2SO4 takes two NH3)
So %N = 1.4 × M × 2(V − V1/2) / m.
The factor 1.4 is just 14 × 100 / 1000, from 14 g of nitrogen per mole and mL-to-L.
Worked example 2: a Kjeldahl numerical you can check
Sample numbers again, but with a real compound so you can check the answer. PubChem lists acetamide as C2H5NO, 59.07 g/mol, so its nitrogen content should be 14.01 / 59.07 = 23.7%.
A 0.295 g sample of acetamide is digested and the ammonia is passed into 50.0 mL of 0.100 M H2SO4. The excess acid needs 50.0 mL of 0.100 M NaOH.
Excess acid = 50.0 / 2 = 25.0 mL
Acid used = 50.0 − 25.0 = 25.0 mL, which is 0.00250 mol H2SO4
Ammonia = 2 × 0.00250 = 0.00500 mol, containing 0.0700 g of nitrogen
%N = 0.0700 / 0.295 × 100 = 23.7%
The formula agrees: 1.4 × 0.100 × 2(50.0 − 25.0) / 0.295 = 23.7%.
When can't you use Kjeldahl's method?
NCERT states that the method does not work for compounds with nitrogen in nitro and azo groups, or nitrogen in a ring such as pyridine, because that nitrogen is not converted to ammonium sulphate under these conditions. For those, use Dumas. Exam questions often ask exactly this.
Real laboratories still use Kjeldahl's idea. Gregorio Cruz describes a common variant in which the ammonia is collected in boric acid instead of standard sulphuric acid, a version recommended by bodies such as ISO and EPA. The Class 11 version with sulphuric acid is the one to learn for exams.
Which mistakes cost the most marks?
Forgetting aqueous tension in Dumas problems, or subtracting it from the wrong pressure.
Mixing STP values. NCERT's Unit 1 uses 22.7 L at 1 bar; this section uses 22,400 mL at 273 K and 760 mm. Use the one your question defines.
Dropping a factor of 2 in Kjeldahl problems, or using V1 instead of V1/2.
Applying Kjeldahl to nitrobenzene or pyridine.
Forgetting that mL must become L in mole calculations.
If you get stuck at the same step every time, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer, which helps you see which factor you missed.
A practice routine
Derive the Kjeldahl formula from the two equations, without looking, until you can do it in two minutes.
Solve NCERT's Problems 8.21 and 8.22, then one sample problem of your own with a known compound, as above.
Revise the qualitative side in our post on Lassaigne's test, then link the percentages to formulas with empirical and molecular formula problems.
Tomorrow, write the Kjeldahl derivation on one side of a page and explain each factor of 2 aloud.
