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Gram-molecular weight, Avogado’s Law, Gram-molecular volume, Formula weight

Def. Molecular weight. The molecular weight of a compound is the sum of the atomic weights of all the individual atoms in the molecule as represented by its formula.

Example. Compute the molecular weight of H₂SO₄.

            2 H = 2 × 1.008 = 2.016

            1 S = 1 × 32.064 = 32.064

            4 O = 4 × 15.994 = 63.998

  Molecular weight =  2.016 + 32.064 + 63.998 = 98.078

Def. Gram-molecular weight (or mole). The number of grams of any substance that is exactly equal to its molecular weight.

Syn. mole

One mole (gram-molecular weight) of any substance contains the same number of molecules as a mole of any other substance. A mole of O₂ contains the same number of molecules as a mole of CO₂ or a mole of H₂SO₄. The number of molecules contained in a mole of any substance is given by Avogado’s number, which is 6.023 ×10²³ molecules.

Formal definition of a mole. A mole is formally defined as the amount of a substance containing the same number of basic units (atoms, molecules, formula units, ions, etc) as there are atoms in 12g of carbon-12. This number is Avogado’s number.

Avogado’s Law. At the same temperature and under the same pressure, equal volumes of gases contain the same number of molecules.

Q. How much space is occupied by a mole of oxygen, O₂, at S.T.P.?

A. One liter of oxygen at S.T.P. weighs 1.43 g and a mole of oxygen is 32 g. Dividing 32 by 1.43 gives 22.4 liters as the amount of space occupied by a mole of oxygen.

By Avogado’s Law 22.4 liters of any other gas will contain the same number of molecules as a mole of oxygen. Thus 22.4 liters of any gas contains exactly one mole of the gas. Consequently if we know the weight of a gas we can compute its gram-molecular weight.

Def. Gram-molecular volume. The volume occupied by one gram-molecular weight (i.e. one mole) of a substance. For all gases the gram-molecular volume is 22.4 liters (at S.T.P.).

Syn. molar volume, normal molar volume.

Formula weight. Many crystalline substances such as sodium chloride, NaCl, and hydrated cupric sulfate, CuSO₄ ∙ 5 H₂O, exist as ions and not in the form of molecules. In such substances the term molecular weight is incorrect and we use the term formula weight for the sum of the atomic weights of the atoms in the formulas.