Dumas Method – Nitrogen % in 60 Sec!

 

❓ Question

Dumas Method – Nitrogen % in 60 Sec!
A quick, exam-oriented breakdown of how Dumas Method helps you find percentage of nitrogen in an organic compound using only the nitrogen gas collected.

---

📝 1️⃣ Dumas Method Concept

Organic compound is heated strongly →
Nitrogen present in compound converts into N₂ gas →
This N₂ is collected over water and its volume, temperature, pressure are measured.

✔ No chemical titration
✔ 100% physical calculation
✔ Based on ideal gas equation

---

📝 2️⃣ Correcting the Pressure (MOST Important Step)

Gas is collected over water, so total pressure =
Pressure of dry N₂ + water vapour pressure.

Corrected pressure:

$$P_{N_2}=P_{\text{total}}-P_{\text{aq. tension}}$$

Given:

$$P_{\text{total}}=715\text{ mmHg},P_{\text{aq. tension}}=15\text{ mmHg}$$

So,

$$P_{N_2}=700\text{ mmHg}$$

---

📝 3️⃣ Ideal Gas Equation (Find moles of N₂)

$$PV=nRT$$

Convert units:

• $P = 700\ \text{mmHg} = 700/760\ \text{atm}$
  

• $V = 50\ \text{mL} = 0.05\ \text{L}$
  

• $R = 0.0821$
  

• $T = 300\ \text{K}$
  

$$n=\frac{PV}{R\mathrm{T}}$$

This gives moles of N₂.

---

📝 4️⃣ Mass of Nitrogen

Nitrogen comes out as N₂, so:

$$\text{Mass of N}=n\times 28\text{ g}$$

(28 g = molar mass of N₂)

---

📝 5️⃣ Percentage of Nitrogen

Sample mass = 292 mg = 0.292 g

$$\mathrm{\%}N=\frac{\text{mass of N}}{0.292}\times 100$$

For the given values, the final answer (nearest integer) =

$$\boxed{{\displaystyle 18\mathrm{\%}}}$$

---

✅ Conclusion & Video Tip

✔ Dumas Method = Gas measurement, not chemistry
✔ Only steps: pressure correct → PV=nRT → mass → %
✔ Works in less than 60 seconds on exam day