Question

An oxide ${{M}_{2}}{{O}_{3}}$ contains 10.3% oxygen by weight. Calculate the atomic weight of M.

Answer 1

Hint: The ratio of the average mass of the atom of a chemical element is called atomic weight or atomic mass. The isotope C-!2 is the standard unit of atomic mass that has been a one-twelfth mass of an atom. Most chemical reactions take place following numerical relationships of atoms by the concept of atomic weight in fundamental chemistry.

Complete step by step answer:
Given that, metal oxide contains 10.3% oxygen by weight of oxide.
Then if metal oxide weight = 100g,
Weight of oxygen in metal oxide = 10.3g
Weight of metal in metal oxide = 89.7g
So, 10.3g of oxygen reacts with 89.7g of metal forms a metal oxide ${{M}_{2}}{{O}_{3}}$
So, 8g of oxygen will react with = $8 X \dfrac{89.7}{10.3}$ = 69.7 g
So equivalent weight of metal = 69.7g
Since the atomic ratio of metal and oxygen = 2:3
Then the valence of metal = 3
Hence, the atomic weight of metal M = equivalent weight of metal X valence of the metal
 = $69.7 X 3$ = 209.1 gram/mol
Therefore, the atomic weight of metal M =209.1 gram/mol

Note: The two isotopes of carbon are C-12 and C-13. Each isotope carbon atom has the same number of protons and electrons, 6. But C-12 has neutrons 6; C-13 has neutrons 7. So, to define the modern atomic weight scale must specify which C atom represents the scale. The standard atomic mass unit is also called an Avogram, which is a unit mass of and weight equal to one gram divided by Avogadro’s number.