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Answer : The molecular mass of a substance expressed in grams is called gram molecular mass or molar mass. A mole is a collection of 6.022 × 1023 particles or a mole is defined as the amount (mass) of a substance containing elementary particles like atoms, molecules or ions in 12 gram of carbon. Avogadro’s number is defined as the number of atoms present in 12 gram (gram atomic mass) of C-13 isotope, i.e., 6.022 × 1023 atoms.
Mole of Molecules- One mole contains 6.022 × 1023 molecules and is equivalent to the gram molecular mass of any given substance. Thus 1 mole of oxygen (O2) contains 6.022 × 1023 molecules and weighs 32 grams. 1 mole of water contains 6.022 × 1023 molecules and weighs 18 gram. It has been noticed that one mole of any gaseous molecule occupies 22.4 dm3 (liter) or 22400 cm3 (ml) at S.T.P. This volume is known as molar volume. Hence, molar volume of a gas is defined as the volume occupied by one mole of a gas at S.T.P.
Mass of one molecule of oxygen:
Mass of oxygen atom= 16 amu
Mass of oxygen (O2) molecule = 16 × 2 = 32 amu
Gram molecular mass of oxygen = 32.0 grams
Number of oxygen molecules in 32.0 gram of O2 = 6.022 × 1023 molecules
Therefore; Mass of one molecule of oxygen = (32.0/ 6.022 × 1023) = 5.314 × 10-23 gram.
Hence; Mass of one molecule of oxygen is 5.314 × 10-23 gram.
