In chemistry, we deal with various types of chemical reactions. To quantify and ensure that we get reproducible results, definite amounts of reactants and products need to be measured. Definition Under the SI system of units, a mole is defined as the amount of a substance (a chemical element or compound), that contains as many elementary entities (molecules, atoms, or ions), as there are atoms in 12 gm of carbon-12. A mole contains exactly 6.023 x 10²³ (Avogadro's number) elementary entities (atoms, molecules, or ions) of that substance. So, think of a mole of a substance as an amount that contains exactly 6.023 x 10²³ molecules and atoms. So, molar mass of a substance is the mass of one mole of that substance. Let us see the calculation method. Keep a periodic table with atomic mass in front of you, along with a calculator. Also, while calculating, you need to know the molecular formula of the concerned compound. Calculation For Elements Molar mass of an element is the atomic weight of the element, expressed in grams/mol. See which element's mass is to be calculated and note its standard atomic weight. Multiply it by 1 g/mol, which is the standard constant. The product of the standard atomic weight of the element with 1 g/mol, will give you the molar mass. For example, the atomic weight of carbon is 12.01. Multiply this value with 1 gm/mol and the product, 12.01 gm/mol, is the molar mass of the element. Calculation For Compounds The molar mass of a compound is a sum of the molar masses of atoms, that make the compound. Here is the calculation procedure. Get the molecular formula of the compound. Note down the different atomic elements that are contained in the molecular formula, along with the number of atoms of each element. Note down the standard atomic weights for each element. Then, after multiplying the standard atomic weights of each element by the number of atoms of each element, sum up all the products. Next, multiply the sum by the constant (1 gm/mol) and you have what you need. Let us see the calculation for water, whose molecular formula is H₂O. Hydrogen has two atoms while oxygen has only one. Therefore, Molar mass of Water (H₂O) = (2 x 1.00794 + 1 x 15.9994 ) x 1 gm/mol = 18.015 gm/mol Calculating this parameter is purely a plug-in problem, once you understand the concept. Just put in the numbers and get the answer.