Answer is: mass of ammonia is 34 grams.
Chemical reaction: 4NH₃ + 7O₂ → 4NO₂ + 6H₂O.
N(O₂) = 2,1·10²⁴.
n(O₂) = N(O₂) ÷ Na.
n(O₂) = 2,1·10²⁴ ÷ 6·10²³ 1/mol.
n(O₂) = 3,5 mol.
From chemical reaction: n(O₂) : n(NH₃) = 7 : 4.
n(NH₃) = 2 mol.
m(NH₃) = n(NH₃) · M(NH₃).
m(NH₃) = 2 mol · 17 g/mol.
m(NH₃) = 34 g.
