number of moles of nitrogen dioxide (NO2) produced when 3 moles of oxygen are produced in the decomposition of nitric acid by light?
?

Equation for the decomposition of HNO₃ by light :
4HNO₃(aq) → 2H₂O(l) + 4NO₂(g) + O₂(g)

According to the above equation, 4 moles of NO₂ is produced when 1 mole of O₂ is produced.

When 3 moles of O₂ is produced, number of moles of NO₂ produced
= (3 mol) × 4
= 12 mol