Q. 29.5 mg of organic compound containing nitrogen was digested according to Kjeldahl’s method and the evolved ammonia was absorbed in 20mL of 0.1 M HCl solution. The excess of the acid required 15mL of 0.1M NaOH solution for complete neutralisation. The percentage of nitrogen in the compound is
A) 59.0
B) 47.4
C) 23.7
D) 29.5
Explanation
(c)
Weight of organic compound =29.5mg
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ NH_{3} + HCl \longrightarrow Na_{4}Cl$
$HCl\ \ \ \ \ \ \ \ \ + \ \ \ \ NaOH\ \ \ \ \ \ \ \ \ \ \longrightarrow NaCl + H_{2}O$
(remaining) $15 \times 0.1\ M$
=1.5 mmol
Total millimole of HCl=2
Millimole used by $NH_{3} = 2 - 1.5 = 0.5$
Weight of $NH_{3}$=0.5 ×17mg=8.5mg
Weight of nitrogen $= \frac{14}{17} \times 8.5mg = 7mg$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \%\ Nitrogen = \frac{7}{29.5} \times 1100 = 23.7\%$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.