Acompound containing nitrogen and oxygen is decomposed in the laboratory and produces 1.78 g of nitrogen and 4.05 g of oxygen, calculate empirical formula

First  step
calculate  the   moles  of   each  element
moles =  mass/molar  mass
 N=  1.78g /  14 g /mol   = 0.127  moles
O= 4.05g/16 g/mol  =  0.253   moles

then  find the  mole  ratio  by  dividing with   the  smallest  mole  number   in    each  element  
that  is   N=  0.127/0.127=   1mole
              O =  0.253/0.127= 2moles
therefore  the  empirical  formula  is  NO2

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