Question

Define the law of multiple proportions. Explain it with two examples. How does this law point to the existence of atoms?

Answer 1

Defining the Law of multiple proportion and examplesWhen two elements combine to form two or more chemical compounds, then the masses of one of the elements which combine with a fixed mass of the other, bear a simple whole number ratio to one another.

Example 1:

Let us assume 2 molecules CO (carbon monoxide) and $C{O}_2$ (carbon dioxide).$CO=12uofcarbon+16uofOxygen.$
$C{O}_2=12ucarbon+32uofOxygen.$

The ratio of the mass of oxygen (which combines with fixed mass of carbon (12 u) ) in the given two compounds is $16:32=1:2$.

Which is simple whole no. ratio. Thus, law of multiple proportions is proved

Example 2:

The masses of oxygen which combine with a fixed mass of Sulphur $\left(32u\right)inS{O}_{3}andS{O}_2$ are 48 u and 32 u respectively. Thus, masses of oxygen bear a simple whole no. ratio of 48 : 32 or 3 : 2 to each other., when combined with fixed mass of sulphur (32 u).

Understanding regarding how law of multiple proportions points to existence of atoms: The law of multiple proportions was based on one of the observations of Dalton's atomic theory. According to the law of multiple proportions whenever two elements form more than one compound, the different masses of one

This law shows that there are constituents which combine in a definite proportion. These constituents may be atoms. Thus, the law of multiple proportions shows the existence of atoms which combine to form molecules