When conc. H2SO4 is added to dry KNO3, brown fumes are evolved. These fumes are due to A. SO3 B. SO2 C. NO2 D. N2O

The correct answer is: C. NO2

When conc. H2SO4 is added to dry KNO3, a vigorous reaction

occurs and brown fumes of nitrogen dioxide (NO2) are evolved. The reaction is as follows:

$$KNO_3 + H_2SO_4 \to KNO_2 + H_2O + NO_2$$

The brown fumes of NO2 are due to the oxidation of nitric

oxide (NO) by the sulfuric acid. The nitric oxide is formed as a by-product of the reaction between the potassium nitrate and the sulfuric acid. The sulfuric acid reacts with the potassium nitrate to form potassium sulfate and nitric oxide. The nitric oxide then reacts with the oxygen in the air to form nitrogen dioxide.

The nitrogen dioxide is a poisonous gas that can cause respiratory problems. It is also a major contributor to acid rain.

The other options are incorrect because they are not formed in the reaction between conc. H2SO4 and dry KNO3.

• Option A: SO3 is sulfur trioxide. It is not formed in the reaction between conc. H2SO4 and dry KNO3.
• Option B: SO2 is sulfur dioxide. It is not formed in the reaction between conc. H2SO4 and dry KNO3.
• Option D: N2O is nitrous oxide. It is not formed in the reaction between conc. H2SO4 and dry KNO3.