N2O3 28 48 2 3 Nitrogen Tetroxide N2O4 28 64 2 4 Nitrogen pentoxide N2O5 28 80 2 5 Note that the proportion of O by weight is in each case a multiple of the first, 16. Also that the proportion by volume of O is a multiple of that in the first compound.
"An Introduction to Chemical Science"
R.P. Williams