The law of multiple proportions, which is a fundamental principle in chemistry, states that when elements combine to form compounds, the ratio of the masses of one element that combines with a fixed mass of another element can be expressed in small whole numbers. This law allows us to determine the relative ratios of different elements in a compound.
However, knowing the ratio of elements in a compound does not provide direct information about the actual number of atoms present. To determine the absolute number of atoms in a sample, including the mass of one atom, we need additional information.
Avogadro's law and the concept of the mole provide a bridge between the macroscopic world (measurable quantities) and the microscopic world (atoms and molecules). Avogadro's law states that equal volumes of gases, under the same temperature and pressure conditions, contain the same number of particles (atoms or molecules). This principle establishes a relationship between the volume of a gas and the number of gas particles.
By using Avogadro's law, scientists can determine the number of gas particles (atoms or molecules) present in a known volume. This relationship, along with the determination of the molar mass (the mass of one mole of a substance), allows for the calculation of the number of atoms or molecules in a given sample.
Therefore, Avogadro's law is necessary to establish the link between measurable quantities (such as volume) and the number of atoms or molecules, providing a way to determine the absolute number of atoms in a sample even without knowing the mass of a single atom.