The molar mass of a chemical compound is the mass, in grams, of 1 mole of the substance.
Step 1: Find molar mass of an element Find the molar mass of an element. In grams, molar mass is numerically equal to the element's atomic weight in atomic mass units, which you can find on the periodic table of the elements.
Step 2: Calculate diatomic elements Remember that seven elements – hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine, are diatomic elements. As pure elements, they form molecules containing two atoms. To find the molar mass of a diatomic element, multiply its atomic weight by two.
Step 3: Consider chemical compounds without subscripts If you're working with a chemical compound – a combination of elements – whose formula does not contain any subscripts, add the molar masses of each of the elements.
TIP: For example, the molar mass of sodium chloride is the molar mass of one atom of sodium (23 grams/mole sodium) plus the molar mass of one atom of chlorine (35.5 grams/mole chlorine), or 58.5 grams/mole sodium chloride.
Step 4: Consider chemical compounds with subscripts If your compound does contain subscripts, multiply the number of atoms of any element or chemical group with a subscript by the subscript value.
TIP: For example, the molar mass of iron chloride is (1 atom times 56 grams/mole iron) plus (2 atoms times 35.5 grams/mole chlorine) equals 127 grams/mole iron chloride.
Step 5: Consider chemical compounds with more complicated formulas If you're working with a compound that has more complicated subscripts, take into account the total number of atoms present in the compound.
TIP: In the case of zinc nitrate, the molar mass is (1 atom times 65 grams/mole zinc) plus (two atoms times 14 grams/mole of nitrogen) plus (six atoms times 16 grams/mole oxygen) equals 189 grams/mole of zinc nitrate.
Step 6: Use the calculation Use molar mass in all kinds of chemistry calculations, including finding the molecular formula of a substance.
FACT: The word molar comes from the Latin word for mass.