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MCQEasyJEE 2023Quantitative Analysis (C, H, N…)

JEE Chemistry 2023 Question with Solution

In Dumas method for the estimation of N2N_2, the sample is heated with copper oxide and the gas evolved is passed over:

  • A

    Ni

  • B

    Copper gauze

  • C

    Pd

  • D

    Copper oxide

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: In Dumas' method, a nitrogen-containing organic compound is heated with CuO\text{CuO} in an atmosphere of CO2\text{CO}_2.

Find: The substance over which the evolved gas is passed.

The reaction described in the solution is:

CxHyNz+(2x+y2)CuO→xCO2+y2H2O+z2N2+(2x+y2)CuC_xH_yN_z + \left(2x + \frac{y}{2}\right)\text{CuO} \rightarrow x\text{CO}_2 + \frac{y}{2}\text{H}_2\text{O} + \frac{z}{2}\text{N}_2 + \left(2x + \frac{y}{2}\right)\text{Cu}

During this process, traces of nitrogen oxides may also form. These nitrogen oxides are reduced back to N2N_2 by passing the gaseous mixture over heated copper gauze.

Therefore, the correct option is B.

Role of Copper Gauze

Given: Nitrogen estimation is being carried out by Dumas' method.

Find: Which material is used after heating with copper oxide.

In this method, the organic compound is oxidized by CuO\text{CuO} to form CO2\text{CO}_2, H2O\text{H}_2\text{O}, and N2\text{N}_2. However, some oxides of nitrogen can be formed as side products.

Heated copper gauze converts these oxides of nitrogen into nitrogen gas. Hence, the evolved gas is passed over copper gauze so that only nitrogen is finally collected for estimation.

So, the correct answer is Copper gauze, i.e. option B.

Common mistakes

  • Choosing copper oxide because it is used first in the method. This is incorrect because CuO\text{CuO} is the oxidizing agent used during heating, whereas the evolved gases are later passed over copper gauze. Distinguish between the heating reagent and the gas-treatment reagent.

  • Ignoring the role of nitrogen oxides. This is incorrect because traces of nitrogen oxides may form and must be reduced to N2N_2 for accurate estimation. Remember that heated copper gauze performs this reduction.

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