To determine the number of molecules in a given mass of a substance, you can utilize Avogadro’s number along with the substance’s molar mass.

To perform this calculation, you need two critical pieces of information: the molar mass of the substance and Avogadro’s number. The molar mass represents the mass of one mole of a substance, typically expressed in grams per mole (g/mol). Avogadro’s number, also referred to as Avogadro’s constant, quantifies the number of particles (which can be atoms, molecules, ions, etc.) contained in one mole of a substance. This constant is approximately $6.022 \times 10^{23}$ particles per mole.

The first step in your calculation is to convert the mass of the substance into moles. You can achieve this by dividing the given mass by the molar mass of the substance. The formula for this conversion is:

Once you have calculated the number of moles, you can then find the total number of molecules by multiplying the number of moles by Avogadro’s number. The corresponding formula is:

For example, consider you have $18$ grams of water (H$_2$O). The molar mass of water is approximately $18$ g/mol. This indicates that you have $1$ mole of water. By multiplying this by Avogadro’s number, you find that there are approximately $6.022 \times 10^{23}$ molecules of water.

Keep in mind that this method is applicable to any substance, provided you know both its molar mass and the mass you are working with. This fundamental concept in chemistry enables us to engage with quantities of substances at the molecular level, even though we typically handle these materials in visible amounts.