College

The Kjeldahl procedure was used to analyze 256 μL of a solution containing 37.9 mg protein/mL. The liberated ammonia was collected in 5 mL of 0.0336 M HCl, and the remaining acid required 6.34 mL of 0.010 M NaOH for complete titration. What is the weight percentage of nitrogen in the protein? MW = 14

Answer :

The weight percentage of nitrogen in the protein is approximately 15.09%.

The steps are:

1. Find the amount of protein in the solution:

Volume of the solution (V) = 256 µL = 0.256 mL

Concentration of protein = 37.9 mg/mL

Convert the volume into mL because the concentration is given in mg/mL.

Amount of protein = 37.9 mg/mL * 0.256 mL

Amount of protein = 9.7024 mg

2. Calculate the moles of HCl originally present:

Volume of HCl = 5 mL = 0.005 L

Molarity of HCl = 0.0336 M

Moles of HCl = Molarity × Volume

Moles of HCl = 0.0336 M × 0.005 L

Moles of HCl = 1.68 × 10⁻⁴ mol

3. Calculate the moles of NaOH used in the back titration:

Volume of NaOH solution = 6.34 mL = 0.00634 L

Molarity of NaOH = 0.010 M

Moles of NaOH = Molarity × Volume

Moles of NaOH = 0.010 M × 0.00634 L

Moles of NaOH = 6.34 × 10⁻⁵ mol

4. Determine the moles of HCl that reacted with ammonia:

The reacted HCl will be the initial moles of HCl minus the moles of NaOH used in the back titration.

Moles of HCl that reacted = 1.68 × 10⁻⁴ mol - 6.34 × 10⁻⁵ mol

Moles of HCl that reacted = 1.046 × 10⁻⁴ mol

5. Calculate the moles of NH₃ liberated:

The moles of NH₃ will be equal to the moles of HCl that reacted since they react in a 1:1 ratio.

Moles of NH₃ = 1.046 × 10⁻⁴ mol

6. Find the grams of nitrogen in the liberated ammonia:

Molar mass of nitrogen (N) = 14 g/mol

Mass of nitrogen = Moles of NH₃ × Molar mass of nitrogen

Mass of nitrogen = 1.046 × 10⁻⁴ mol × 14 g/mol

Mass of nitrogen = 1.4644 × 10⁻³ g = 1.4644 mg

7. Calculate the weight percentage of nitrogen in the protein:

Percentage of nitrogen = (Mass of nitrogen / Mass of protein) × 100%

Percentage of nitrogen = (1.4644 mg / 9.7024 mg) × 100%

Percentage of nitrogen = 15.09%