Molar mass is the mass of a substance that has 6.022 × 10²³atoms (Avogadro's number). The mass number of an element is equal to the molar mass of the element . The molar mass of a chemical compound is the mass when the compound has 6.022 × 10²³molecules. Molar mass of Chlorine is 35.45 due to consideration of avg molar mass by including mass of its all isotopes.