Two oxides of a metal contain 27.6% and 30% oxygen, respectively. If the formula of the first compound is M3O4, find the formula of the second compound.

Given,
     In the case of the 1^st oxide,  Mass of oxygen = 27.6
     Therefore,
          Mass of metal = 100 - 27.6 = 72.4
     In the case of the 2^nd oxide,  Mass of oxygen = 30
     Therefore,
          Mass of metal = 100 - 30 = 70
Also,
     Formula of the 1^st oxide is  M₃O₄
Therefore,
     Number of atoms of metal in the 2^nd oxide    = (3/72.4) x 70
                                                                       =  2.9
     Number of atoms of oxygen in the 2^nd oxide = (4/27.6) x 30
                                                                       =  4.35
Thus,
     Ratio of metal : oxygen in the 2^nd oxide = 2.9 : 4.35 or  ≈ 2 : 3
Hence,
     The Formula of the 2^nd oxide is M₂O₃.