Estimation Of Nitrogen (kjeldahl's Method)

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In Kjeldahl's method, 0.30 g of an organic compound liberated ammonia that exactly neutralised 30 mL of 0.1 N sulphuric acid; what is the percentage of nitrogen in the compound?

RankGuru · Accepted Answer

Kjeldahl's method estimates nitrogen by heating the organic compound with concentrated sulphuric acid so that the nitrogen is converted into ammonium sulphate. On treating with excess alkali, ammonia is liberated, absorbed in a known volume of standard acid, and the acid that reacts is determined. The percentage of nitrogen is given by %N = (1.4 x normality of acid x volume of acid in mL) / (mass of compound in g). Substituting the data: %N = (1.4 x 0.1 x 30) / 0.30 = 4.2 / 0.30 = 14%. The factor 1.4 arises from 14 (equivalent mass of nitrogen) divided by 10 when concentrations are in normality and volume in millilitres. The value 9.33% comes from wrongly dividing by 0.45 g or mismatching the volume. The value 4.2% mistakenly omits division by the mass, leaving only the numerator scaled wrongly. The value 28% doubles the result, as if two equivalents of nitrogen were counted per acid equivalent, which is incorrect. This formula is exactly the NCERT Kjeldahl relation. Plausibility check: a 14% nitrogen content is reasonable for a typical nitrogenous organic compound, confirming the answer.

Estimation Of Nitrogen (kjeldahl's Method)

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