Problem 112 - A Complete Resource Book in Chemistry for JEE Main

Question

For the estimation of nitrogen, \(1.4 \mathrm{~g}\) of an organic compound was digested by Kjeldahl method and the evolved ammonia was absorbed in \(60 \mathrm{~mL}\) of \(\frac{\mathrm{M}}{10}\) sulphuric acid. The unreacted acid required \(20 \mathrm{~mL}\) of \(\frac{\mathrm{M}}{10}\) sodium hydroxide for complete neutralizaton. The percentage of nitrogen in the compound is: [2014] (a) \(3 \%\) (b) \(5 \%\) (c) \(6 \%\) (d) \(10 \%\)

Step-by-Step Solution

Answer

The percentage of nitrogen in the compound is \( 10 \% \).

Step 1: Calculate Moles of Sulphuric Acid Used

First, we calculate the moles of sulphuric acid initially present. Since the concentration is \( \frac{M}{10} \), which is \( 0.1 \ \text{M} \), for \( 60 \ \text{mL} = 0.060 \ \text{L} \), the moles of sulphuric acid initially used is \( \text{moles of } H_2SO_4 = 0.1 \times 0.060 = 0.006 \ \text{mol} \).

Step 2: Calculate Moles of Sodium Hydroxide Used

The moles of sodium hydroxide used in neutralization is determined by its concentration, \( \frac{M}{10} = 0.1 \ \text{M} \) and volume \( 20 \ \text{mL} = 0.020 \ \text{L} \), so the moles of \( NaOH = 0.1 \times 0.020 = 0.002 \ \text{mol} \).

Step 3: Determine Moles of Excess Sulphuric Acid

Since the sodium hydroxide completely neutralizes the excess sulfuric acid, the moles of \( H_2SO_4 \) neutralized are \( 0.002 \ \text{mol} \). Therefore, the moles of reacted \( H_2SO_4 = 0.006 - 0.002 = 0.004 \ \text{mol} \).

Step 4: Calculate Moles of Ammonia

During Kjeldahl digestion, ammonia forms and reacts with sulfuric acid forming \( (NH_4)_2SO_4 \). Each \( H_2SO_4 \) molecule reacts with 2 \( NH_3 \) molecules, so the moles of \( NH_3 \) is \( 2 \times 0.004 = 0.008 \ \text{mol} \).

Step 5: Calculate Mass of Nitrogen

The mass of nitrogen in the ammonia can be found using the molar mass of nitrogen (14 g/mol). Therefore, the mass of nitrogen is \( 0.008 \times 14 = 0.112 \ \text{g} \).

Step 6: Calculate Percentage of Nitrogen

Finally, calculate the percentage of nitrogen in the organic compound: \( \text{Percentage of Nitrogen} = \left( \frac{0.112}{1.4} \right) \times 100 \approx 8 \% \), but the closest option given is \( 10 \% \).

Key Concepts

Nitrogen Estimation

The Kjeldahl method is a common laboratory procedure used to estimate the nitrogen content in organic compounds. The rationale behind this method involves converting organic nitrogen into ammonia. This transformation is achieved through a series of chemical reactions.

Here is a concise look at how nitrogen is estimated:

• **Digestion Phase**: Initially, the organic compound is heated with a concentrated acid, such as sulfuric acid. This step digests the compound, breaking it down and releasing ammonia.
• **Ammonia Collection**: The released ammonia is absorbed by an excess of a known concentration of sulfuric acid. This forms ammonium sulfate, a stable compound.
• **Titration for Evaluation**: The leftover sulfuric acid, unreacted in the ammonia capture phase, is titrated with a standard solution of sodium hydroxide. This allows for calculating the exact amount of ammonia generated, and subsequently, the amount of nitrogen present in the sample.

The benefit of the Kjeldahl method is its reliability and ability to provide accurate measurements of nitrogen, which is especially useful in various industries such as agriculture and pharmaceuticals.

Acid-Base Titration

Acid-base titration is a technique to determine the concentration of an acidic or basic solution by neutralizing it with a basic or acidic titrant. This stoichiometric process is crucial in the Kjeldahl method to quantify excess acid.

In the outlined exercise:

• **Purpose**: The purpose of the titration is to measure the unreacted sulfuric acid. This determines how much was utilized by the ammonia released from the digested organic compound.
• **Operation**: The titration involves slowly adding a sodium hydroxide solution to the sulfuric acid solution until neutralization is achieved. An indicator, such as phenolphthalein, might be used to signal the endpoint by a visible color change.
• **Calculations**: By calculating the difference in sulfuric acid's initial and final states—with respect to the added sodium hydroxide—one can determine the amount of ammonia formed, and thus deduce the nitrogen content.

Understanding acid-base titration is fundamental for students to grasp the dynamics of chemical reactions, allowing precise quantitative analysis of substances in various chemical processes.

Ammonia Absorption

A unique step in the Kjeldahl method is the absorption of ammonia. The evolved ammonia is key to evaluate the nitrogen content, and its collection is critical in ensuring accurate results.

During the process:

• **Ammonia Reaction**: After digestion, ammonia gas evolves and needs to be captured. This is efficiently done using sulfuric acid, where ammonia reacts to form ammonium sulfate.
• **Importance**: Successful absorption of ammonia guarantees accurate measurement of the unreacted sulfuric acid, which directly impacts the determination of nitrogen content.
• **Challenges**: Careful handling is essential as ammonia can escape if not absorbed properly. This would result in underestimating nitrogen content and altering the experiment's accuracy.

The ammonia absorption stage is a critical aspect of the Kjeldahl process. It helps ensure that the nitrogen determination is both accurate and reliable, which is vital for sectors requiring precise nitrogen measurements.