Estimation Of Nitrogen (kjeldahl's Method)
In Kjeldahl's estimation, 1.4 g of an organic compound liberated ammonia that was exactly neutralised by 20 mL of 1.0 N hydrochloric acid. Calculate the percentage of nitrogen in the compound.
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About This Question
- Subject
- chemistry
- Chapter
- principles related to practical chemistry
- Topic
- estimation of nitrogen (kjeldahl's method)
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
20%
Kjeldahl's method estimates nitrogen by digesting the compound with concentrated sulphuric acid to convert nitrogen into ammonium sulphate, then liberating ammonia with alkali and absorbing it in a known amount of standard acid; the acid neutralised is equivalent to the ammonia, and hence to the nitrogen. The milliequivalents of acid neutralised equal volume times normality: 20 mL × 1.0 N = 20 meq of HCl, which equals 20 meq of NH_3 and therefore 20 meq of nitrogen. Mass of nitrogen = 20 × 14 / 1000 = 0.28 g, since the equivalent mass of nitrogen here equals its atomic mass 14. Percentage of nitrogen = (0.28 / 1.4) × 100 = 20%. Option 28% wrongly uses the nitrogen mass as the percentage directly without dividing by sample mass. Option 10% would arise from halving the acid equivalents. Option 40% would result from using a 0.7 g sample instead of 1.4 g. This calculation follows the standard NCERT formula %N = 1.4 × N_acid × V_acid / mass. Plausibility check: a 20% nitrogen content is chemically reasonable for many amides and amines, confirming the magnitude is sensible.
This medium difficulty chemistry question is from the chapter principles related to practical chemistry, covering the topic of estimation of nitrogen (kjeldahl's method). It appeared in the 2025 exam.
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