Problem 71 - A Complete Resource Book in Chemistry for JEE Main

Question

\(0.5 \mathrm{~g}\) of an organic compound in \(\mathrm{Kjeldahl}\) experiment needs \(29 \mathrm{ml}\) of \(\frac{\mathrm{N}}{5} \mathrm{H}_{2} \mathrm{SO}_{4}\) for complete neutralisation of \(\mathrm{NH}_{3}\). The \(\mathrm{N} \%\) is? (a) \(16.2\) (b) \(8.1\) (c) \(24.3\) (d) \(32.4\)

Step-by-Step Solution

Answer

The nitrogen percentage is 16.2%, option (a).

Step 1: Write Down Known Values and Formula

We begin by identifying the known values: the mass of the organic compound is 0.5 g, and the volume of the acid used is 29 ml. The acid concentration is \( \frac{N}{5} \). The formula for the percentage of nitrogen is given by \[ \text{N} \% = \frac{\text{Mass of Nitrogen}}{\text{Mass of Compound}} \times 100 \].

Step 2: Calculate the Normality of Acid

The given normality of \( \mathrm{H}_{2}\mathrm{SO}_{4} \) is \( \frac{N}{5} \). This indicates the normality of the acid used in the reaction is \( 0.2 \text{ N} \).

Step 3: Calculate the Amount of NH3 Neutralized

Given the volume of \( \mathrm{H}_{2}\mathrm{SO}_{4} \) used is 29 ml, and its normality is 0.2 N, convert volume to liters: \( V = 0.029 \text{ L} \). Calculate the moles of \( \mathrm{H}_{2}\mathrm{SO}_{4} \): \( = N \times V = 0.2 \times 0.029 = 0.0058 \text{ equiv} \). Assuming complete conversion, \( \text{moles of NH}_3 = \text{moles of H}_2\mathrm{SO}_4 = 0.0058 \text{ moles} \).

Step 4: Calculate the Mass of Nitrogen

The reaction implies 1 mole of \( \mathrm{NH}_{3} \) gives 1 mole of \( \mathrm{N} \) and has a molar mass of \( 14 \text{ g/mol} \) for N. Therefore, the mass of nitrogen is \( 0.0058 \times 14 = 0.0812 \text{ g} \).

Step 5: Calculate the Nitrogen Percentage

Using the formula, substitute the values calculated: \( \text{N} \% = \frac{0.0812}{0.5} \times 100 = 16.24\% \). We can approximate this to 16.2\%.

Key Concepts

Organic Compound Analysis

Organic compound analysis is a critical process to determine the composition of organic materials. This involves breaking down the compound into simpler substances to analyze the elements or functional groups present. The Kjeldahl method is a specific technique for analyzing nitrogen content within a compound. It involves digesting the organic material in sulfuric acid, converting nitrogen to ammonia, and then measuring this ammonia. This is crucial for applications in agriculture, food science, and environmental science to assess protein content and nitrogen levels. Students focusing on organic chemistry often see Kjeldahl as a classic method that combines both qualitative and quantitative aspects of chemical analysis.

Nitrogen Percentage Calculation

Calculating the nitrogen percentage in an organic compound is pivotal to understanding its composition. This involves a series of mathematical steps based on experimental data obtained from the Kjeldahl method.

• First, the mass of the compound tested is noted.
• The experiment then measures the nitrogen-containing ammonia produced, and with the aid of acid-base titration, determines the amount of nitrogen.

To find the nitrogen percentage, you use the formula:\[ \text{N} \% = \left( \frac{\text{Mass of Nitrogen}}{\text{Mass of Compound}} \right) \times 100 \]This calculation allows scientists to assess the total nitrogen present in the compound, illustrated by determining that 0.0812 g of nitrogen in a 0.5 g compound leads to 16.2% nitrogen.

Acid-Base Titration

Acid-base titration is a fundamental chemical technique used to determine the concentration of a specific acid or base in a solution. In the context of the Kjeldahl method, titration plays an integral role. After the digestion phase, where nitrogen in the organic compound is converted to ammonia, this ammonia potentially contributes to a controlled acid-base reaction.

• A known volume and concentration of standard acid, like sulfuric acid, is used to neutralize the ammonia derived from the compound.
• The endpoint of this reaction is detected by a pH indicator or a pH meter.

From the volume of acid standard used, combined with its known concentration, the amount of ammonia and thus the nitrogen content can be calculated. This allows precise determination of nitrogen levels, crucial for ensuring accurate results in nitrogen percentage calculations.