Discussion Forum

For the esimation  of nitrogen, 1.4 g of an organic compound was digested  by Kjeldahl's  method and the evolved ammonia  was absorbed in 60 mL  of   $\frac{M}{10}$ sulphuric acid. The unreacted  acid required 20 mL of   $\frac{M}{10}$ sodium hydroxide  for complete  neutralisation . The percentage  of nitrogen in the compound

Answer:

Explanation:

 This problem is based on the estimation of percentage of N in organic compound using Kjeldahl's method. Use the concept of stoichiometry  and follow the steps given below to solve the problem

 (i)  Write the balanced chemical reaction for the conversion of N present in organic compound to ammonia, ammonia to ammonia sulphate and ammonium sulphate to sodium sulphate.

 (ii) Calculate  millimoles (m moles ) of N present in organic compound followed by mass of N present in organic compound using the concept of stoichiometry

(iii) At last calculate  % of N present  in organic compound using formula

 %  of N=  Mass of N X 100/ mass of organic compound.

Mass of organic compound =1.4 g

 Let it contain x mmole of N atom.

  Organic compound →  NH₃ (x mmole)

$ 2NH_{3}+H_{2}SO_{4}\rightarrow (NH_{4})_{2}SO_{4}$        ............(i)

                6m mole initially taken

$H_{2}SO_{4}+2NaOH\rightarrow Na_{2}SO_{4}+2H_{2}O$   .............(ii)

 2mmoles NaOH reacted.

 Hence mmoles of   $H_{2}SO_{4}$ reacted inEq.(ii)=1

$\Rightarrow$ mmoles of   $H_{2}SO_{4}$ reacted in Eq.(i)=6-1=5 mmoles

$\Rightarrow$   mmoles of   $NH_{3}$ in Eq.(i)= 2 x 5= 10.mmoles

$\Rightarrow$ mmoles of N atom  in the organic compound=10mmoles

  $\Rightarrow$  Mass of N=   $10\times 10^{-3}\times14=0.14 g$

 % of N= Mass of N present in organic compound/ Mass of organic compound  x 100

$\Rightarrow$       % of N=   $\frac{0.14}{1.4}\times100=10$%