Problem 111 - A Complete Resource Book in Chemistry for JEE Main

Question

\(29.5 \mathrm{mg}\) of an organic compound containing nitrogen was digested according to Kjeldahl's method and the evolved amomnia was absorbed in \(20 \mathrm{~mL}\) of \(0.1 \mathrm{M}\) HCl solution. The excess of the acid required \(15 \mathrm{~mL}\) of \(0.1 \mathrm{M} \mathrm{NaOH}\) solution for complete neutralization. The percentage of nitrogen in the compound is \([\mathbf{2 0 1 0}]\) (a) \(59.0\) (b) \(47.4\) (c) \(23.7\) (d) \(29.5\)

Step-by-Step Solution

Answer

The percentage of nitrogen in the compound is 23.7%.

Step 1: Calculate Moles of HCl Added

First, calculate the moles of HCl initially added to the evolved ammonia. Use the formula for moles \( n = C \times V \), where \( C \) is concentration and \( V \) is volume. The concentration of HCl is \(0.1\ M\) and the volume is \(20\ mL\) or \(0.02\ L\). Thus, the moles of HCl initially present is \(0.1\ M \times 0.02\ L = 0.002\ mol\).

Step 2: Calculate Moles of NaOH Used in Neutralization

Next, calculate the moles of NaOH used to neutralize the excess HCl. Using \( n = C \times V \) again, where the concentration of NaOH is \(0.1\ M\) and the volume is \(15\ mL\) or \(0.015\ L\), the moles of NaOH used is \(0.1\ M \times 0.015\ L = 0.0015\ mol\).

Step 3: Calculate Moles of HCl Reacted with Ammonia

Determine the moles of HCl that reacted with the evolved ammonia by subtracting the moles of NaOH from the moles of HCl initially present. Thus, the moles of HCl that reacted is \(0.002\ mol - 0.0015\ mol = 0.0005\ mol\).

Step 4: Calculate Moles of Ammonia Formed

Since each mole of HCl reacts with a mole of ammonia \((NH_3)\), the moles of ammonia produced is equal to the moles of HCl that reacted. Hence, the moles of ammonia formed is \(0.0005\ mol\).

Step 5: Calculate Mass of Nitrogen in the Compound

Calculate the mass of nitrogen in the compound using the molar mass of nitrogen \( (14.01\ g/mol)\). Therefore, the mass of nitrogen is \(0.0005\ mol \times 14.01\ g/mol = 0.007005\ g = 7.005\ mg\).

Step 6: Calculate Percentage of Nitrogen in the Compound

Finally, calculate the percentage of nitrogen in the compound using the formula: \(\text{Percentage} = \frac{\text{mass of nitrogen}}{\text{mass of compound}} \times 100\%\). Therefore, the percentage of nitrogen is \(\frac{7.005\ mg}{29.5\ mg} \times 100\% = 23.7\%\).

Key Concepts

Nitrogen Estimation

Nitrogen estimation is a crucial method used in chemistry to determine the nitrogen content in organic compounds. The preferred technique for this purpose is Kjeldahl's method, which is widely used due to its reliability and simplicity. This method involves digesting the organic compound with sulfuric acid, converting nitrogen into ammonium sulfate. Then, the ammonia is liberated through the addition of a base and is captured in a known quantity of an acid. By knowing the amount of acid neutralized by the ammonia, chemists can back-calculate the nitrogen content. This allows for a precise estimation of nitrogen within a compound, which is essential for studies in agricultural sciences, food industry testing, and various research fields.

Organic Chemistry

Organic chemistry is the study of the structure, properties, and reactions of organic compounds, which are based on carbon chains and include elements such as nitrogen, oxygen, and sulfur. The Kjeldahl method often focuses on organic nitrogen, common in amino acids and proteins. Understanding the presence of nitrogen in these compounds can reveal valuable information about their protein content and nutritional value. In laboratories, organic chemists use techniques like nitrogen estimation to validate the composition of organic compounds. This contributes to the development and refinement of pharmaceuticals, polymers, and various industrial chemicals.

Analytical Chemistry

Analytical chemistry involves the techniques used to determine the composition of materials. Nitrogen estimation through Kjeldahl's method is a prime example of how analytical procedures are applied to learn about a compound's makeup.

• Titration: This precise method involves adding a reagent to react quantitatively with the compound being analyzed, often to determine a specific constituent, such as nitrogen.
• Accuracy Confirmation: Various checks, such as reagent purity and instrument calibration, are essential to ensure an accurate analysis. Techniques may include blank tests and standard solutions.

The skills and knowledge within analytical chemistry are crucial for the advancement of scientific research and development in fields ranging from environmental studies to pharmaceuticals.

Molar Calculations

Molar calculations are fundamental to the application of many chemical methods, including Kjeldahl's nitrogen estimation. In this context, they allow for the conversion of volumes and concentrations of solutions in moles, providing a pathway to determine the quantity of nitrogen. For instance, in determining nitrogen percentage, the key steps include:

• Determining Moles of Reagents: Calculate the number of moles of hydrochloric acid and sodium hydroxide using the formula \( n = C \times V \) (where \( C \) is concentration and \( V \) is volume).
• Reacting Moles Equivalence: Understand that the moles of acid reacting with ammonia directly equate to the moles of ammonia produced.
• Mass and Percentage Calculations: Once the moles of nitrogen are known, they are converted into mass using the molar mass (\( 14.01\ g/mol \) for nitrogen) and then calculated into percentage form by dividing by the sample mass and multiplying by 100%.

Molar calculations are instrumental for accuracy in chemistry experiments and ensure reliable results for research and industry applications.