Class 9 Chemistry Atoms and Molecules Molecular mass

Molecular mass

Molecular mass of a substance is defined as the sum o the atomic masses of all the atoms present in a molecule of a substance. Therefore the relative mass of a molecule is expressed as atomic mass units (amu).


Let us calculate the molecular mass of H2O. we will find it out by following process.

  • We know the atomic mass of hydrogen is 1u and that of oxygen is 16u.
  • Molecular mass of H2O containing two hydrogen atoms and single oxygen atom is

2 X 1 + 1 X 16 = 18u


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