The Dumas method is used to determine the percentage of nitrogen in an organic compound.
Dumas Method – JEE / NEET Chemistry
1. Definition
The Dumas method is used to determine the percentage of nitrogen in an organic compound.
2. Principle
A known mass of an organic compound containing nitrogen is heated strongly with excess copper oxide (CuO). Carbon and hydrogen are oxidised to carbon dioxide (CO₂) and water (H₂O), while nitrogen is converted into nitrogen gas (N₂).
Hot copper helps convert nitrogen oxides into nitrogen gas (N₂).
3. Formula for Percentage of Nitrogen
When the volume of nitrogen gas is measured at STP:
- V = Volume of nitrogen gas at STP in mL
- m = Mass of the organic compound in grams
- 28 = Molar mass of nitrogen gas (N₂) in g/mol
- 22400 mL = Molar volume of an ideal gas at STP (0°C and 1 atm)
Simplified Formula
% N = V / (8 × m)
Here, V must be in mL and m must be in grams.
4. Numerical Example
Question: 0.5 g of an organic compound gives 112 mL of nitrogen gas at STP. Calculate the percentage of nitrogen.
Given:
Mass of compound (m) = 0.5 g
Volume of nitrogen gas (V) = 112 mL
Solution:
Using the formula:
% N = V / (8 × m)
% N = 112 / (8 × 0.5)
% N = 112 / 4
Answer: % N = 28%
5. Important Points for JEE / NEET
| Point | Remember |
|---|---|
| Purpose | Estimation of nitrogen |
| Reagent | Copper oxide (CuO) |
| Gas measured | Nitrogen (N₂) |
| Molar mass of N₂ | 28 g/mol |
| Molar volume at STP | 22.4 L/mol |
| Simplified formula | % N = V / (8 × m) |
6. Important Exam Tip
If nitrogen gas is given at conditions other than STP, first convert its volume to STP using the combined gas equation:
(P₁V₁) / T₁ = (P₂V₂) / T₂
Temperature must be expressed in kelvin, and pressure units must be consistent.
Conclusion: The Dumas method is an important method for estimating nitrogen in organic compounds and is useful for JEE and NEET Chemistry preparation.
