In Dumas method for the estimation of N2, the sample is heated with copper oxide and the gas evolved is passed over:

Step 1: Understand the Dumas' Method Apparatus

In Dumas' method, the sequence of processes in the combustion tube is:

1. Organic compound is heated with excess $\ce{CuO}$ → produces $\ce{CO2}$, $\ce{H2O}$, and $\ce{N2}$ (plus any $\ce{NOx}$ oxides if N is present)
2. The gas mixture exits the CuO section
3. The gases are passed over copper gauze (reduced copper)
4. Then passed through KOH solution (which absorbs $\ce{CO2}$ and $\ce{H2O}$)
5. Only $\ce{N2}$ remains and is collected in the nitrometer

Step 2: Why Copper Gauze?

During heating with CuO, some nitrogen oxides ($\ce{NO}$, $\ce{NO2}$) may be formed: $\ce{N + excess CuO -> NOx}$

The gases are passed over hot copper gauze to reduce these nitrogen oxides back to $\ce{N2}$: $\ce{NOx + Cu (gauze) -> N2 + CuO}$

This ensures that ALL nitrogen is collected as $\ce{N2}$ and the volume measured accurately.

Step 3: Evaluate Each Option

| Option | Reason it would be wrong | |---|---| | (a) Ni | Nickel is not used in this step | | (b) Copper gauze ✅ | Reduces NOₓ → N₂; ensures complete N₂ collection | | (c) Pd | Palladium is not used in Dumas' method | | (d) Copper oxide | CuO is already in the first stage; excess CuO would re-oxidise the gases |

Step 4: Conclusion

Answer: (b) Copper gauze

Key Points to Remember:

• Dumas' method sequence: CuO oxidation → copper gauze (reduce NOₓ) → KOH absorption → nitrometer
• Copper gauze ensures NOₓ → N₂ (prevents under-collection of N₂)
• The nitrometer collects N₂ over KOH solution (not water, since CO₂ must be absorbed)
• Final measurement: volume of N₂ at temperature T and pressure P