Molar mass (molecular weight) of a substance is the mass in grams of 1 mole of compound.
The molar mass of known substance is obtained by summing the masses of the atoms.
Methane is a molecular compound—its components are molecules. Many substances are ionic—they contain simple ions or polyatomic ions. Examples are NaCl (contains Na⁺ and Cl⁻) and CaCO₃ (contains Ca²⁺ and CO₃²⁻). Because ionic compounds do not contain molecules, we need a special name for the fundamental unit of these materials. Instead of molecule, we use the term formula unit. Thus CaCO₃ is the formula unit for calcium carbonate, and NaCl is the formula unit for sodium chloride.