Molecular Weight. The weight of a molecule is the sum of the weights of the atoms of which it is made. The unit of weight is the dalton, one-twelfth the weight of an atom of 12 C. Thus the molecular weight (MW) of water is 18 daltons. Molecular Weight. Molecules are composed of several atoms, for example a carbon dioxide molecule (CO 2) is made up of 1 carbon atom and 2 oxygen atoms. The molecular weight (or molecular mass or relative molecular mass (RMM)) is the sum of the atomic weights of all the atoms in the molecule. Example 1. Calculate the molecular weight of carbon Molar mass and formula weight are commonly used as numerical approximations for molecular weight. Molar mass is the mass of a substance containing 1 mol of molecules; it has the unit of g/mol. The molar mass of a compound with a given chemical formula can be calculated as the sum of the atomic weights of all atoms appearing in the formula and Popular answers (1) Molecular mass and Molecular weight is not the same, although they use interchangeably. The molecular mass ( m) is the mass of a given molecule: it is measured in daltons (Da Fortunately, this is not difficult, as the molar mass of an element is exactly the same as the atomic weight of that element. A table of atomic weights can be used to find the molar mass of elements (this information is often included in the periodic table). For example, the atomic weight of oxygen is 16.00 amu, so its molar mass is 16.00 g/mol. If we knew that 1 cup of cheese = 50. g then we could use it in a math equation, this would be our molar mass. 1.0 cup of cheese 50.0 g of cheese (1) (1) 1.0 cup of cheese 50.0 g of cheese. Let's pretend that I have 250. g of cheese that I take out of the refrigerator and I want to know how many cups of cheese I have. The mass in grams of 1 mole of substance is its molar mass. Due to the use of the same reference substance in defining the atomic mass unit and the mole, the formula mass (u) and molar mass (g/mol) for any substance are numerically equivalent (for example, one H 2 O molecule weighs approximately 18 u and 1 mole of H 2 O molecules weighs Molar Mass (Atomic Weight) of a Formula Unit. The atomic weight, molecular weight, or formula weight of one mole of the fundamental units (atoms, molecules, or groups of atoms that correspond to the formula of a pure substance) is the ratio of its mass to 1/12 the mass of one mole of C 12 atoms, and being a ratio, is dimensionless. QZqD.