Estimation of nitrogen: There are two methods for the estimation of nitrogen (i) Dumas method and (ii) Kjedahl's method.
i. Dumas method: A known mass of compound is heated with copper oxide (CuO) in an atomsphere of `CO_(2)`, which gives free nitrogen along with `CO_(2) and H_(2)O`
`C_(x)H_(y)N_(z) + (2x + y//2) CuO rarr xCO_(2) + y//2 (H_(2)O) + z//2 (N_(2)) + (2x + y//2) CU`
The gaseous mixture is passed over a heated copper gauze which converts traces of nitrogen oxides formed to `N_(2)`. The gaeous mixture is collected over an aqueous solution of KOH which absorbs `CO_(2)`, and nitrogen is callected in the upper part of the graduated tube.
ii. Kjeldahl's method: A known mass of organic compound (0.5 gm) is mixed with `K_(2)SO_(4)` (10 gm) and `CuSO_(4)`. (1.0 gm) or a drop of mercury (Hg) and conc. `H_(2)SO_(4)` (25ml), and heated in Kjeldahl's flask. `CuSO_(4)` or Hg acts as a catalyst, while `K_(2)SO_(4)` raises the boiling point of `H_(2)SO_(4)`. The nitrogen in the organic compound is quantitatively converted to ammonium sulphate. The resulting mixture is then distilled with excess of NaOH solution and the `NH_(3)` evolved is passed into a known but excess volume of standard HCl or `H_(2)SO_(4)`. The acid left unused is estimated by titration with some standard alkali. The amount of acid used against `NH_(3)` can be thus be known and from this the percentage of nitrogen is calculated.
(a) `C + H + S underset(H_(2)SO_(4))overset("conc.")rarr CO_(2) + H_(2)O + SO_(2)`
(b) `N underset(H_(2)SO_(4))overset("conc.")rarr (NH_(4))_(2)SO_(4)`
(c) `(NH_(4))_(2) SO_(4) + 2NaOH rarr Na_(2)SO_(4) + 2NH_(3) + 2H_(2)O`
(d) `2NH_(3) + H_(2)SO_(4) rarr (NH_(4))_(2) SO_(4)`
(iii) This method is not applicable to compounds containing N in intro and azo groups, and N present in the ring (e.g., pyridine) as N of these compounds does not change to `(NH_(4))_(2)SO_(4)` (ammonium sulphate) under these reaction conditions
0.4gm of an organic compound was treated according to Kjeldahl's method. The ammonia evolved was absorbed in 50ml of 0.5M `H_(3)PO_(3)`. The residual acid required 30 ml of 0.5M `Ca(OH)_(2)`. Find the percentage of `N_(2)` in the compound

Options

A. 20
B. 50
C. 70
D. 45

Answer

C