Two oxides of metal contain 27.6% and 30.0% of oxygen respectively. If

1.	Two oxides of metal contain 27.6% and 30.0% of oxygen respectively. If the formula of the first oxide is M3O4, find that of the second.
Answer» In the first oxide, oxygen = 27.6, metal = 100 - 27.6 = 72.4 parts by mass. As the formula of the oxide is M₃O₄, this means 72.4 parts by mass of metal = 3 atoms of metal and 4 atoms of oxygen = 27.6 parts by mass. In the second oxide, oxygen = 30.0 parts by mass and metal = 100 - 30 = 70 parts by mass. But 72.4 parts by mass of metal = 3 atoms of metal. ∴ 70 parts by mass of metal = 3/72.4 × 70 atoms of metal = 2.90 atoms of metal. Also, 27.6 parts by mass of oxygen = 4 atoms of oxygen. ∴ 4.35 atoms of oxygen. Hence, ratio of M: O in the second oxide = 2.90 : 4.35 = 1:1.5 = 2:3. ∴ Formula of the metal oxide is M₂O₃.

Two oxides of metal contain 27.6% and 30.0% of oxygen respectively. If the formula of the first oxide is M3O4, find that of the second.

Answer»

In the first oxide, oxygen = 27.6, metal = 100 - 27.6 = 72.4 parts by mass.

As the formula of the oxide is M₃O₄, this means 72.4 parts by mass of metal = 3 atoms of metal and 4 atoms of oxygen = 27.6 parts by mass.

In the second oxide, oxygen = 30.0 parts by mass and metal = 100 - 30 = 70 parts by mass.

But 72.4 parts by mass of metal = 3 atoms of metal.

∴ 70 parts by mass of metal = 3/72.4 × 70 atoms of metal = 2.90 atoms of metal.

Also, 27.6 parts by mass of oxygen = 4 atoms of oxygen.

∴ 4.35 atoms of oxygen.

Hence, ratio of M: O in the second oxide = 2.90 : 4.35 = 1:1.5 = 2:3.

∴ Formula of the metal oxide is M₂O₃.