What is the equation used to calculate the number of moles of a substance?

The equation used to calculate the number of moles of a substance is indeed mass divided by molar mass. This relationship is based on the definition of a mole, which is the amount of substance that contains as many entities (like atoms, molecules, or ions) as there are in 12 grams of carbon-12.

In this context, mass refers to the total mass of the substance you have, typically measured in grams, while molar mass is the mass of one mole of that substance, expressed in grams per mole (g/mol). Therefore, when you divide the mass of the substance by its molar mass, you derive the number of moles, allowing you to relate the macroscopic sample size (mass) to the microscopic scale (number of entities).

The other options do not accurately represent the correct relationship for finding the number of moles. For instance, multiplying mass by molar mass would yield a result inconsistent with physical laws, and the other methods mentioned do not follow from the mole concept defined in chemistry. Thus, understanding the correct equation is essential for performing calculations related to chemical quantities in stoichiometry and other areas of chemistry.