Q.During estimation of nitrogen present in an organic compound by Kjeldahl's method, the ammonia evolved from 0.5 g of the compound neutralised 10 mL of 1 M H₂SO₄. Find out the percentage of nitrogen in the compound.

Concept: Kjeldahl’s method — In Kjeldahl’s method, the ammonia ($NH_3$) evolved from the organic compound is absorbed in a known volume of standard acid. The amount of acid neutralised gives the nitrogen content.

Steps:

1. Moles of $H_2SO_4$ neutralised:

   $10\ \text{mL}$ of $1\ \text{M}\ H_2SO_4$ contains

   $\text{moles} = \frac{10}{1000} \times 1 = 0.01\ \text{mol}$.

2. Moles of $NH_3$ that reacted:

   $2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO_4$

   So $1\ \text{mol}\ H_2SO_4$ reacts with $2\ \text{mol}\ NH_3$.

   $\therefore$ Moles of $NH_3 = 2 \times 0.01 = 0.02\ \text{mol}$. …

Why this approach works

Kjeldahl’s method is a classic way to estimate nitrogen in organic compounds. The compound is digested with concentrated sulphuric acid, which converts all nitrogen into ammonium sulphate. When you add excess alkali, ammonia gas is liberated. This ammonia is then trapped in a known volume of standard acid. The amount of acid that remains unreacted is found by back-titration, or — as in this problem — the acid is completely neutralised by the ammonia. The key relationship is:

Why twice? Because each mole of sulphuric acid ($\text{H}_2\text{SO}_4$) provides two moles of $\text{H}^+$ ions, and each mole of ammonia ($\text{NH}_3$) accepts one $\text{H}^+$ to form $\text{NH}_4^+$. So the neutralisation reaction is:

Once you know the moles of ammonia, you know the moles of nitrogen (since each $\text{NH}_3$ contains one N atom). Multiply by the atomic mass of nitrogen (14 g/mol) to get the mass of nitrogen, then find the percentage relative to the sample mass.

Step-by-step solution

1. Find the moles of $\text{H}_2\text{SO}_4$ used

We are told that 10 mL of 1 M $\text{H}_2\text{SO}_4$ is completely neutralised by the ammonia.

Molarity (M) = moles per litre. So:

2. Calculate the moles of ammonia ($\text{NH}_3$) produced

From the balanced equation: $2\text{NH}_3 + \text{H}_2\text{SO}_4 \rightarrow (\text{NH}_4)_2\text{SO}_4$, we see that 1 mole of $\text{H}_2\text{SO}_4$ reacts with 2 moles of $\text{NH}_3$.

Therefore:

3. Relate ammonia to nitrogen

Each molecule of ammonia contains one nitrogen atom. So:

4. Find the mass of nitrogen …