Question

In the Kjeldahl's method for estimation of nitrogen present in a soil sample, ammonia evolved from 0.75 gm of sample neutralized 10 mL of 1 M H₂SO₄, The percentage of nitrogen in the soil is:

• A. 45.33
• B. 35.33
• C. 43.33
• D. 37.33

Answer 1

The Correct Option is D

To determine the percentage of nitrogen in the soil sample using Kjeldahl's method, we'll follow these steps:

1. The Kjeldahl's method involves digesting the soil sample with concentrated sulfuric acid, which converts organic nitrogen into ammonium sulfate. The ammonium is then distilled off as ammonia.
2. Ammonia evolved is reacted with a standard acid, here 1 M H₂SO₄.
3. Equation for reaction:
   \text{2NH}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{(NH}_4)_2\text{SO}_4
4. From the balanced equation, 2 moles of NH₃ react with 1 mole of H₂SO₄.
5. Given: Volume of H₂SO₄ = 10 mL = 0.01 L and Molarity = 1 M
6. Calculate moles of H₂SO₄ used:
   \text{Moles of H}_2\text{SO}_4 = \text{Molarity} \times \text{Volume in L}
   = 1 \times 0.01 = 0.01 \text{ moles}
7. Using stoichiometry, \text{Moles of NH}_3 = 2 \times 0.01 = 0.02 \text{ moles}
8. Mass of nitrogen in ammonia:
   Molar mass of nitrogen (N) = 14 g/mol
   Total nitrogen mass = 0.02 \times 14 = 0.28 \text{ g}
9. Now calculate the percentage of nitrogen in the original soil sample:
   \text{Percentage of N} = \left(\frac{\text{Mass of N}}{\text{Mass of soil sample}}\right) \times 100
   = \left(\frac{0.28}{0.75}\right) \times 100 \approx 37.33\%

Therefore, the percentage of nitrogen in the soil sample is 37.33%, which is the correct answer.