Discovering that the volume of a gas was directly proportional to the number of particles it contained was crucial in establishing the formulas for simple molecules at a time (around 1811) when the distinction between atoms and molecules was not clearly understood. In particular, the existence of diatomic molecules of elements such as $$H_2$$, $$O_2$$, and $$Cl_2$$ was not recognized until the results of experiments involving gas volumes was interpreted.

Early chemists mistakenly assumed that the formula for water was $$HO$$, leading them to miscalculate the molecular weight of oxygen as 8 instead of 16. However, when chemists found that an assumed reaction of

$H+Cl→HCl$

yielded twice the volume of $$HCl$$, they realized hydrogen and chlorine were diatomic molecules. The chemists revised their reaction equation to be

$H_2+Cl_2→2HCl.$

When chemists revisited their water experiment and their hypothesis that

$HO \rightarrow H + O$

they discovered that the volume of hydrogen gas consumed was twice that of oxygen. By Avogadro's Law, this meant that hydrogen and oxygen were combining in a 2:1 ratio. This discovery led to the correct molecular formula for water ($$H_2O$$) and the correct reaction

$2H_2O→2H_2+O_2.$

## Summary

• The number of molecules or atoms in a specific volume of ideal gas is independent of size or the gas' molar mass.
• Avogadro's Law is stated mathematically as follows: $$\frac{V}{n} = k$$, where $$V$$ is the volume of the gas, n is the number of moles of the gas, and k is a proportionality constant.
• Volume ratios must be related to the relative numbers of molecules that react; this relationship was crucial in establishing the formulas of simple molecules at a time when the distinction between atoms and molecules was not clearly understood.

## Contributors

• Boundless (www.boundless.com)