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   Atomicity of ammonia is the number of atoms in a molecule - it is 4 .

   Avogadro;s law is that  V / n = constant  provided the pressure and temperature are constant.    If P an T are same, then 
      (P / T) * (V / n)  for all the gases is same.

   to find atomicity we need to take n mole of N2 and 3 n moles of Hydrogen at the same Temperature, and pressure.  We find volumes V1 of N2 and,  V2 of  Hydrogen.  They combine to form ammonia.  We measure the volume of the gas by isothermally making the end pressure same as before the reaction.  Let the volume of ammonia be V3.

      (P/T * V1 / n) = (P/ T) * V2 / (3*n)  =  (P/T) * V3 / n3
  So from the volume of ammonia produced, we find the number of moles n3 of ammonia produced.
       Thus we have 2 n  moles of hydrogen atoms and 2 * 3 n moles of atoms of hydrogen.  The total number of atoms = 8 n moles of atoms.
     divide this by n3.    We get  8 n / n3  ratio.  This ratio gives the atomicity .

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