Molecular weight is defined as the sum of the atomic weights, or atomic mass units, of the atoms in one molecule of a molecular substance. A molecular substance can be a gas or a liquid composed of more than one atom of an element, or any chemical compound. Molecular weight is the same as "formula weight." Chemists use a quantity called the "mole" as a convenient measure of substances in chemical calculations. A mole is defined as the "Avogadro number" of atoms of an element or molecules of a substance.
The Avogadro Number
The Avogadro number is 6.02 x (10 raised to the power 23). In chemistry, the significance of this number is seen in the relationship between the masses of individual atoms and their atomic weights, and between the masses of individual molecules and their molecular weights.
Atomic Weight and Gram-Atomic Weight
The atomic weight of an element is defined as its weight relative to the element carbon, which by international convention is assigned atomic weight 12.000. "Gram-atomic weight" is related to the Avogadro number and is clarified by dividing the atomic weight of an element by the actual weight, in grams, of one atom of the element. As an example, by dividing the atomic weight of oxygen (O), by the weight of a single oxygen atom, the Avogadro number is obtained: 16.00 g/1 gram-atomic weight x 1 atom/26.6 x (10 raised to the power -24) g = 6.02 x (10 raised to the power 23) atoms per gram-atomic weight of oxygen.
Molecular Weight and Gram-Molecular Weight
Continuing with the example of oxygen, the concepts discussed above are further clarified by considering molecular oxygen, O2, which is oxygen gas. Because O2 has 2 atoms of oxygen, its molecular weight is 16.00 x 2 = 32.0. One "gram-molecular weight" of oxygen is the weight of the Avogadro number of oxygen molecules, or 32.0 grams. Another example is water, H2O, which has a molecular weight of 18.0 and a gram-molecular weight of 18.0 grams.
Molar Mass Vs Molecular Weight
Molar mass is the mass, usually expressed in grams or kilograms, of one mole of an element or chemical compound. This is distinguished from molecular weight, which is defined in terms of the "relative" weights of atoms. For example, the molar mass of elemental oxygen can be expressed as 16 grams or 16 kilograms, depending on which unit is convenient for a particular calculation. In chemical engineering, where large quantities of materials are considered, pounds and even tons are used. Thus you can express a mass of oxygen in "pound-moles" or "ton-moles." It is important to note here that the terms weight and mass are used interchangeably. Although technically incorrect, it has no consequence in chemical calculations.