Atomic Mass And Molecular Mass

Atomic mass:

Atomic Mass and Molecular Mass

Atomic Mass and Molecular Mass

Every particle of matter has some amount of mass associated with it whether small or large. Everything is made up of atoms. Mass of an atomic particle is called the atomic mass. This is commonly expressed as per the international agreement in terms of a unified atomic mass unit (amu). It can be best defined as \(\frac{1}{12}\) of the mass of a carbon-12 atom in its ground state. The mass of an atom can be accounted by the sum of mass of protons and neutrons which is almost equal to the atomic mass. This small change is due to the binding energy mass loss.

1 amu = 1.66 ×10−24 g.

The atomic weight of an atom is a dimensionless number when it is divided by unified atomic weight or Daltons. This is called the relative isotropic mass.  The atomic masses of elements vary from 1.008 amu for hydrogen up to 250 amu for elements which have a very high atomic number. Mass of molecules can be determined by adding the average atomic mass of each atom in the molecule.


Molecular mass:

Molecular mass is the amount of mass associated with a molecule. It is also called as molecular weight. It can be calculated by adding the mass of each atom multiplied by the number of atoms of the element present in the molecule. For example, water is made up of 2 hydrogen atoms and 1 oxygen atom. Mass of water molecule is equal to the average atomic mass of hydrogen multiplied by two plus the atomic mass of oxygen. Molecular mass of elements depends upon the constituent atoms of the molecule.

The molecular weight of molecules can be determined by following methods:

  • Mass spectrometry: This method is generally used in determining the mass of small molecules. This is reported as monoisotropic mass.
  • Hydrodynamic method: The weight is determined as per Mark-Houwink relations. This method requires calibration hence it is also described as relative molecular weight determination method.
  • Static Light Scattering: Molecular weight is determined from the amount of light scattered using the Zimm method.

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Practise This Question

The mass of Sodium in 11.7 g of Sodium chloride is