Define the law of multiple proportions. Explain it with two examples. How does this law point to the existence of atoms?
'Law of multiple proportions' was first studied by Dalton in 1803 which may be defined as follows
When 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 ratio to one another.
e.g., hydrogen combines with oxygen to form two compounds, namely, water and hydrogen peroxide.
$$\begin{aligned} &\begin{aligned} & \underset{2 \mathrm{~g}}{\text { Hydrogen }}+\underset{32 \mathrm{~g}}{\text { Oxygen }} \longrightarrow \underset{34 \mathrm{~g}}{\text { Hydrogen peroxide }} \end{aligned}\\ &\underset{2 \mathrm{~g}}{\text { Hydrogen }}+\underset{32 \mathrm{~g}}{\text { Oxygen }} \longrightarrow \underset{34 \mathrm{~g}}{\text { Hydrogen peroxide }} \end{aligned}$$
Here, the masses of oxygen (i.e., 16 g and 32 g ) which combine with a fixed mass of hydrogen $(2 \mathrm{~g})$ bear a simple ratio, i.e., $16: 32$ or $1: 2$.
As we know that, when compounds mixed in different proportionation, Then they form different compounds. In the above examples, when hydrogen is mixed with different proportion of oxygen, then they form water or hydrogen peroxide.
It 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 into molecules.
A box contains some identical red coloured balls, labelled as $A$, each weighing 2 g . Another box contains identical blue coloured balls, labelled as $B$, each weighing 5 g . Consider the combinations $A B, A B_2$, $A_2 B$ and $A_2 B_3$ and show that law of multiple proportions is applicable.
| Combnation | Mass of A (g) | Mass of B (g) |
|---|---|---|
| AB | 2 | 5 |
| AB$$_2$$ | 2 | 10 |
| A$$_2$$B | 4 | 5 |
| A$$_2$$B$$_3$$ | 4 | 15 |
Mass of $B$ which is combined with fixed mass of $A$ (say 1 g ) will be $2.5 \mathrm{~g}, 5 \mathrm{~g}, 1.25 \mathrm{~g}$ and 3.75 g . They are in the ratio $2: 4: 1: 3$ which is a simple whole number ratio. Hence, the law of multiple proportions is applicable.