Ostwald's method is based upon catalytic oxidation of NH3 by atmospheric oxygen.
4NH3(g)+5O2(g)→500K,9barPt/Rhgaugecatalyst(A)4NO(g)+6H2O(g)
Nitric oxide thus formed combines with oxygen giving NO2.
2NO(g)+O2(g)(B)⇌2NO2(g)
Nitrogen dioxide so formed, dissolves in water to give HNO3.
3NO2(g)+H2O(l)→2HNO3(aq)+NO(g)
∴ A and B are NO and NO2 respectively.