Question

The weights of two elements which combines with one another are in the ratio of their:
a) Atomic weight
b) Molecular weight
c) Equivalent weight
d) None of the above

Answer 1

Hint: The law of multiple proportions comes to play in this question. The law of multiple proportions states that when two elements are able to combine with each to form more than one compound, the weights of the element that combine with the fixed weight of the other element are in the ratio of small whole numbers.

Complete answer:
To find the ratio of the weights of the elements, atomic weight of each element is to be determined.
Let us take an example involving carbon and oxygen. Two known forms in which carbon and oxygen combine are carbon monoxide $\left( {CO} \right)$ and carbon dioxide $\left( {C{O_2}} \right).$ Taking oxygen to have the fixed weight in both compounds, in carbon dioxide, for $32g$ of oxygen present there is $12g$ of carbon. By dividing $32g$ by $12g$ , the mass ratio of oxygen to carbon is $2.66$ to $1$ , while in carbon monoxide, for $16g$ of oxygen present there is $12g$ of carbon and the mass ratio is $1.33$ to $1.$ So, the ratio of oxygen in the two compounds is $1:2$ , which is a small whole number ratio.
Therefore, the weights of two elements which combine with one another are in the ratio of their a) Atomic weight.

Note:
The law of multiple proportions is a law of stoichiometry. It can only be applied to compounds containing the same elements. It is an extension of the law of definite proportion (also called Proust’s law), which states that a given chemical compound always contains its component elements in fixed ratio.