Using the chemical equation: 4no2(g) + o2(g) + 2h2o(l) = 4hno3 (aq) : calculate the theoretical yield in grams of nitric acid when 2.0 moles nitrogen dioxide react with 2.0 moles of oxygen gas.

Stoichiometry of NO₂ to O₂ is 4:1
4 mol of NO₂ reacts with 1 mol of O₂
we need to first find the limiting reactant. limiting reactant is the reactant that is fully consumed during the reaction and amount of product formed depends on the amount of limiting reactant present.
If limiting reactant is O₂, there are 2 mol of O₂, then NO₂ present should be 8 mol. However only 2 mol of NO₂ is present, therefore NO₂ is the limiting reactant.
the stoichiometry of NO₂ to HNO₃ is 4:4 therefore 1:1
number of NO₂ moles reacted = number of HNO₃ moles 
Number of HNO₃ moles = 2 mol
mass of HNO₃ = 2 mol x 63 g/mol
                        = 126 g