Question

While estimating the nitrogen present in an organic compound by Kjeldahl’s method, the ammonia evolved from 0.25g of the compound neutralized 2.5mL of 2M H₂SO₄. The percentage of nitrogen present in organic compound is ________

Hint:

Use the equivalents method: normality of the acid equals its molarity times basicity, and at the neutralization point, milliequivalents of acid always equal milliequivalents of the base. From there, convert moles of nitrogen straight into mass.

Answer 1

Correct Answer: 56

To determine the percentage of nitrogen in the organic compound using Kjeldahl’s method, follow these steps: 

1. Calculate the moles of H₂SO₄ used.
   • Molarity (M) = 2 M
   • Volume (V) = 2.5 mL = 0.0025 L
   • Moles of H₂SO₄ = M × V = 2 × 0.0025 = 0.005 mol
2. Determine the moles of NH₃ neutralized. H₂SO₄ reacts with NH₃ in a 1:2 ratio:
   • N₂/NH₃ reaction: 1 mole H₂SO₄ neutralizes 2 moles of NH₃.
   • Moles of NH₃ = 2 × Moles of H₂SO₄ = 2 × 0.005 = 0.01 mol
3. Calculate the mass of nitrogen in the moles of NH₃.
   • Molar mass of N = 14 g/mol
   • Mass of N = 0.01 mol × 14 g/mol = 0.14 g
4. Compute the percentage of nitrogen in the compound.
   • Sample mass = 0.25 g
   • Percentage of N = (Mass of N / Sample mass) × 100% = (0.14 / 0.25) × 100% = 56%

The calculated nitrogen percentage is 56%, which falls within the given range of 56,56.