Answer:
Duma's Method: It can be used
for all nitrogenous compounds. A nitrogeneous compound when heated strongly
with cupric oxide, gives nitrogen gas.
\[{{C}_{x}}{{H}_{y}}{{N}_{z}}+\left(
2x+\frac{y}{2} \right)CuO\xrightarrow{\Delta
}xC{{O}_{2}}+\frac{y}{2}{{H}_{2}}O+\]\[\frac{z}{2}{{N}_{2}}+\left(
2x+\frac{y}{2} \right)Cu\]
Now, we use
the relation \[\left( PV=\frac{w}{m}RT \right)\] to calculated the mass of
nitrogen produced.
Percentage
composition of nitrogen in the given
\[\frac{Mass\,of\,nitrogen}{Mass\,of\,compound}\times
100\]
Kjeldahls
Method
The method
is based on the fact that when the nitrogenous compound is heated with
concentrated sulphuric acid in presence of copper sulphate, the nitrogen
present in the compound is quantitatively converted to ammonium sulphate.
Organic
compound + conc. \[{{H}_{2}}S{{O}_{4}}\]\[\xrightarrow[\Delta ]{CuS{{O}_{4}}+{{K}_{2}}S{{O}_{4}}}{{(N{{H}_{4}})}_{2}}S{{O}_{4}}\]
The ammonium
sulphate so formed is decomposed by heating with excess of alkali and the
ammonia evolved is absorbed in known excess volume of a standard acid\[({{H}_{2}}S{{O}_{4}})\].
The excess acid left after neutralisation with ammonia is estimated by back
titration with standard alkali. The percentage of nitrogen is then calculated from
the amount of ammonia.
Note :
Kjeldahls method is not used in case of nitro
compounds.
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