What it does
To find how much nitrogen is in an organic compound, chemists turn the nitrogen into something easy to measure.
- Dumas method: heat the compound with copper(II) oxide. Its nitrogen comes off as N₂ gas, which is collected and measured. Convert the volume to STP (273 K and 760 mm Hg), and 22 400 mL of N₂ weighs 28 g.
- Kjeldahl method: heat the compound with concentrated sulfuric acid so its nitrogen becomes ammonium sulfate, then drive off ammonia with NaOH and catch it in a known amount of acid. The acid used up tells you how much NH₃ there was. If extra acid is used, the leftover is titrated with NaOH (a back titration).
The page works through every step with your numbers and draws the apparatus.
It follows the quantitative analysis part of Class 11 chemistry (organic chemistry: some basic principles and techniques). The mole bridge page practises the mass–mole–volume steps used here, the formula writer helps with formulas, and the IUPAC namer names the compounds you analyse.
How to use it
- Pick a method.
- Dumas: type the mass of compound, the volume of nitrogen and the temperature and pressure it was measured at. If the gas was collected over water, type the aqueous tension (the pressure of water vapour at that temperature) and it is taken off the pressure.
- Kjeldahl: type the mass, choose the acid (H₂SO₄ catches two NH₃ per formula, HCl one), its molarity and volume. Tick Back titration and type the NaOH used if the acid was in excess.
- Read the steps and the answer, and compare it with the known compounds.
Worked example
Dumas. 0.30 g of compound gives 50 mL of dry N₂ at 300 K and 715 mm Hg. V₀ = 715 × 50 × 273 ÷ (760 × 300) = 42.81 mL at STP. Mass of N₂ = 28 × 42.81 ÷ 22 400 = 0.05351 g. % N = 0.05351 ÷ 0.30 × 100 = 17.84%.
Kjeldahl, direct. The ammonia from 0.50 g of compound neutralises 10 mL of 0.5 M H₂SO₄: H⁺ = 2 × 0.5 × 10 = 10 mmol = 10 mmol NH₃ = 0.14 g N, so % N = 28%.
Kjeldahl, back titration. 1.0 g of compound; ammonia passed into 50 mL of 0.5 M H₂SO₄ (50 mmol H⁺); the leftover acid needs 60 mL of 0.5 M NaOH (30 mmol). Ammonia used 20 mmol, so N = 0.28 g and % N = 28%.
Check yourself
Three questions with fresh numbers: a Dumas calculation, a Kjeldahl calculation and which compounds Kjeldahl cannot handle. Nothing is saved.
FAQ
Why multiply by 273 and divide by 760? That is the gas law P₁V₁/T₁ = P₀V₀/T₀ rearranged to give the volume at STP.
Why does H₂SO₄ count twice? Each H₂SO₄ gives two H⁺, and each NH₃ takes one.
Is this from my textbook? It covers the standard school topic with its own examples and pictures. It is an independent study aid, not part of NCERT.
Where the numbers come from
- Standard quantitative analysis: Dumas method (V₀ = P₁V₁ × 273 / (760 T₁), 28 g of N₂ in 22 400 mL at STP) and Kjeldahl method (% N = 1.4 × basicity × M × V / m, with back titration). School atomic masses (N = 14). Written for fibub; no textbook text, figures or exercises are used.
- Checked against independent hand calculations in Python (Dumas dry and over water; Kjeldahl direct, back titration and with HCl; % N of urea and glycine from their formulas).
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.