A metal oxide is reduced by heating it in a stream of hydrogen.It is found that after compelete reduction 3.15g of the oxide have yielded 1.05g of the metal.We may conclude that

consider +1 and +2 oxidation states for metals

case 1 :

M₂O + H₂  -- > 2M  +H₂O

2x+16                 2x

so, its , 2*3.15/(2x+16) = 1.05/x

 or x= 4 i.e., the compound may be Helium(but helium being inert gas doesn't for oxide)

case 2 :

MO + H₂  -- > M  +H₂O

x+16                 x

so, its , 3.15/(x+16) = 1.05/x

 or x = 8 (but there is no element with atomic weight of 8)

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Simply apply the relationship between equivalent wt. And mass of element i.e.Mass of element/eq.of element=mass of oxid