The mass of oxygen consumed is calculate as follows
write the equation for reaction
4NH3 + 5O2 = 4NO + 6H2O
calculate the moles of ammonia =mass/molar mass
= 6.3 g/ 17 g/mol= 0.37moles
by use of mole ratio between NH3 to O2 which is 4:5 the moles of O2 is therefore= 0.37 x5/4 = 0.463 moles
mass of O2 consumed is therefore = moles xmolar mass
= 0.463 moles x 32 g/mol= 14.82 grams