Question

A sample of \(0.50 \mathrm{~g}\) of an organic compound was treated according to Kjeldahl's method. The ammonia evolved was absorbed in \(50 \mathrm{ml}\) of \(0.5 \mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}\). The residual acid required \(60 \mathrm{~mL}\) of \(0.5 \mathrm{M}\) solution of \(\mathrm{NaOH}\) for neutralisation. Find the percentage composition of nitrogen in the compound.

Short answer

The nitrogen content in the compound is 56%.

Step-by-step solution

Determine Total Acid Moles

First, we need to calculate the total moles of the sulfuric acid initially present. Given that we have \(50 \text{ mL}\) of \(0.5 \text{ M}\) H₂SO₄, we can use the formula for moles: \(\text{moles} = \text{molarity} \times \text{volume (in L)}\).\[\text{Total moles of H}_2\text{SO}_4 = 0.5 \times \frac{50}{1000} = 0.025 \text{ moles}\]

Account for Residual Acid

Next, we find the moles of sulfuric acid remaining after reaction with ammonia by neutralizing it with NaOH. With \(60 \text{ mL}\) of \(0.5 \text{ M}\) NaOH, the moles of NaOH used are:\[\text{Moles of NaOH} = 0.5 \times \frac{60}{1000} = 0.03 \text{ moles}\]Since NaOH reacts with H₂SO₄ in a 2:1 ratio, the moles of unused H₂SO₄ is:\[\text{Unused moles of H}_2\text{SO}_4 = \frac{0.03}{2} = 0.015 \text{ moles}\]

Calculate Moles of Ammonia

The moles of ammonia that neutralized some of the sulfuric acid is found by subtracting the moles of unused sulfuric acid from total moles:\[\text{Moles of } \text{H}_2\text{SO}_4 \text{ that reacted with NH}_3 = 0.025 - 0.015 = 0.01 \text{ moles}\]Since each mole of H₂SO₄ reacts with 2 moles of ammonia, the moles of ammonia evolved is:\[\text{Moles of NH}_3 = 2 \times 0.01 = 0.02 \text{ moles}\]

Calculate Mass of Nitrogen

Knowing the moles of ammonia, we can determine the mass of nitrogen using the molar mass of nitrogen (14 g/mol):\[\text{Mass of N} = 0.02 \times 14 = 0.28 \text{ grams}\]

Determine Percentage Composition

Finally, calculate the percentage composition of nitrogen in the compound using the initial sample mass (0.50 g):\[\text{Percentage of N} = \left(\frac{0.28}{0.50}\right) \times 100 = 56\%\]

Key concepts

Understanding Percentage Composition

Percentage composition in chemistry refers to the relative amount of each element in a chemical compound. To determine the percentage composition, you calculate how much of a certain element is present in a compound with respect to the total mass of the compound. In the context of Kjeldahl's method, we are specifically interested in finding out the percentage of nitrogen.

The formula used to calculate percentage composition is given by:

• \[ \text{Percentage of Element} = \left( \frac{\text{Mass of the Element in the Compound}}{\text{Total Mass of the Compound}} \right) \times 100 \]

In our exercise, we have calculated the mass of nitrogen in the compound to be 0.28 grams. Knowing the total mass of the organic compound is 0.50 grams, we plug these into the formula. This kind of calculation helps chemists and researchers to understand the proportion of a specific element in a compound, which can be crucial when analyzing the composition of unknown substances.

Nitrogen Determination via Kjeldahl's Method

Kjeldahl's method is widely used for determining the nitrogen content in organic compounds. This analytical technique is especially helpful in the agricultural, food, and chemical industries.

The process involves several key steps:

• First, the sample is digested with a concentrated acid, typically sulfuric acid, which breaks down the organic material and converts nitrogen into ammonium sulfate.
• Next, the ammonium ions are liberated as ammonia gas by adding a strong base, like sodium hydroxide.
• The evolved ammonia is then absorbed in a known volume of acid. In our exercise, it was absorbed in 50 ml of 0.5 M sulfuric acid.
• Subsequently, the amount of acid that's not reacted with ammonia is determined by back-titration using a base, such as NaOH.

This method is accurate and has been in use for a long time due to its reliability. It's important because understanding nitrogen content helps determine the protein levels in different substances, guiding decisions in fields like nutrition and agriculture.

Analyzing Organic Compounds

Organic compounds are integral to life as they make up the essential molecules for biological processes. Analyzing these compounds is crucial in numerous scientific fields.

The analysis can be qualitative or quantitative:

• Qualitative analysis identifies the elements or functional groups present in a compound. Techniques like infrared spectroscopy or nuclear magnetic resonance are often used for this purpose.
• Quantitative analysis looks to measure the exact amount of each element. Kjeldahl's method, as used in our problem, is a quantitative technique for measuring nitrogen.

Understanding the makeup of organic compounds is vital for developing new materials, medicines, and food products. By accurately determining the composition, scientists can manipulate the structure of compounds for desired properties and effects. This foundational knowledge thus supports innovation and safety in product development.