“ A ” obtained by Ostwald's method involving air...

Ostwald's method is based upon catalytic oxidation of ${NH}_{3}$ by atmospheric oxygen.

$4{NH}_{3} ( g ) + 5{O}_{2} ( g ) \mathop{\to}_{500K, 9bar}^{Pt / Rhgaugecatalyst}\mathop{4NO ( g )}_{( A )} + 6{H}_{2}O ( g )$

Nitric oxide thus formed combines with oxygen giving ${NO}_{2}$.

$2NO ( g ) + {O}_{2} ( g ) \mathop{⇌ 2{NO}_{2} ( g )}_{( B )}$

Nitrogen dioxide so formed, dissolves in water to give ${HNO}_{3}$.

$3{NO}_{2} ( g ) + {H}_{2}O ( l ) \to 2{HNO}_{3} ( aq ) + NO ( g )$

$∴$ A and B are $NO$ and ${NO}_{2}$ respectively.