Question

In Dumas method for the estimation of nitrogen, 0.25 g of an organic compound gave 40 mL of nitrogen collected at 300 K temperature and 725 mm of Hg pressure. If the aqueous tension at 300 K is 25 mm of Hg, what is the percentage of nitrogen in the compound (approximately)?

• A. 15.76
• B. 17.36
• C. 18.2
• D. 16.8

Answer 1

The correct option is D 16.8

Pressure of pure $N_2$ (excluding pressure from the water vapour)= 725-25 = 700 mm of Hg
Using the ideal gas equation,
PV = nRT
$\frac{700}{760}\times 40\times {10}^{-3}$= $n\times 0.0821\times 300$
$n=1.5\times {10}^{-3}$
So, the mass of $N_2$ is= $1.5\times 28\times {10}^{-3}$=0.042 g
So, the percentage of N = $\frac{0.042}{0.25}\times 100=16.8$%