Atomic and molecular masses.

Atomic  and  molecular  masses. 

Atomic  and  molecular  masses  :  You  know  about  the  terms  atoms  and  molecules.  Thus it is appropriate here to understand what we mean by atomic and molecular masses.
Atomic  Mass  :  Every  element  has  a  characteristic atomic mass. Atomic mass is the mass of an atom. It is actually very very small. For example, the mass of one hydrogen atom is  1.6736 × 10^-24g. This is very small quantity and not easy to measure.

In  the  present  system,  mass  of  an  atom  is determined relative to the mass of a carbon -  12  atom  as  the  standard  and  this  has  been  agreed upon in 1961. In this system, an atom of  carbon-12  is  assigned  a  mass  of  exactly  12.00000  atomic  mass  unit  (amu) and  all  other  atoms  of  other  elements  are  given  a  relative  atomic  mass,  to  that  of  carbon  -  12.  The  atomic  masses  are  expressed  in  amu. One amu is defined as a mass exactly equal to  one  twelth  of  the  mass  of  one  carbon-12  atom. Later on the exact value of atomic mass unit in grams was experimentally established. 

 

Recently,   amu   has   been   replaced   by   unified mass unit called dalton (symbol 'u' or 'Da'), 'u' means unified mass.

Molecular Mass : Molecular mass of a substance is the sum of average atomic masses of  the  atoms  of  an  element  which  constitute  the molecule. Molecular mass of a substance is the mass of one molecule of that substance relative  to  the  mass  of  one  carbon-12  atom.  It  is  obtained  by  multiplying  average  atomic  mass  of  each  element  by  the  number  of  its  atoms and adding them together.For  example,  the  molecular  mass  of  carbon  dioxide (CO2) is

= 1(average atomic mass of C) + 2 (average atomic mass of O)
=  1 (12.0 u) + 2 (16.0 u)  =  44.0 u
Some   more   examples   of   calculations   of   molecular mass.