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Question:

Concentrated nitric acid, upon long standing, turns yellow-brown due to the formation of: N2O4, NO, NO2, N2O

NO2

N2O

NO

N2O4

Solution:

The decomposition of HNO3 is represented by the equation: 4HNO3 → 2H2O + 4NO2 + O2
Due to the formation of nitrogen dioxide, it turns yellow-brown.
Hence, option B is correct.