As with all these problems, we ASSUME a #100*g# mass of compound, and then we interrogate the molar quantities of each element....
#"Moles of carbon"=(64.26*g)/(12.01*g*mol^-1)=5.35*mol.#
#"Moles of hydrogen"=(8.39*g)/(1.00794*g*mol^-1)=8.32*mol.#
#"Moles of nitrogen"=(8.33*g)/(14.01*g*mol^-1)=0.595*mol.#
#"Moles of oxygen"=(19.02*g)/(16.00*g*mol^-1)=1.19*mol.#
And given these molar quantities, we divide thru by the SMALLEST molar quantity to give an #"empirical formula"#, the simplest whole number ratio defining constituent atoms in a species, of...
#C_((5.35*mol)/(0.595*mol))H_((8.32*mol)/(0.595*mol))N_((0.595*mol)/(0.595*mol))O_((1.19*mol)/(0.595*mol))-=C_9H_14NO_2#
And since we know that ....
#"molecular formula"=nxx"empirical formula"#
#335*g*mol^-1=nxx(168.1*g*mol^-1)#..
Clearly #n~=2#...and #"molecular formula"=C_18H_28N_2O_4#.
