Make the formula of nitrogen oxide if the mass fraction of nitrogen in it is 30% and oxygen is 70%.

Given:
NxOy
ω (N in NxOy) = 30%
ω (O in NxOy) = 70%

To find:
NxOy -?

Decision:
1) Let m (NxOy) = 100 g;
2) m (N in NxOy) = ω (N in NxOy) * m (NxOy) / 100% = 30% * 100/100% = 30 g;
3) n (N in NxOy) = m (N in NxOy) / M (N) = 30/14 = 2.1 mol;
4) m (O in NxOy) = ω (O in NxOy) * m (NxOy) / 100% = 70% * 100/100% = 70 g;
5) n (O in NxOy) = m (O in NxOy) / M (O) = 70/16 = 4.4 mol;
6) x: y = n (N in NxOy): n (O in NxOy) = 2.1: 4.4 = 1: 2;
7) Unknown substance – NO2.

Answer: Unknown substance – NO2.