Question

One method of analyzing amino acids is the van Slyke method. The characteristic amino groups (−NH₂) in protein material are allowed to react with nitrous acid, HNO₂, to form N₂ gas. From the volume of the gas, the amount of amino acid can be determined. A 0.0604-g sample of a biological sample containing glycine, CH₂(NH₂)COOH, was analyzed by the van Slyke method and yielded 3.70 mL of N₂ collected over water at a pressure of 735 torr and 29 °C.

CH₂(NH₂)CO₂H+HNO₂⟶CH₂(OH)CO₂H+H₂O+N₂

What was the percentage of glycine in the sample?

a) 23.4%
b) 55.6%
c) 12.1%
d) 34.8%

Answer 1

The percentage of glycine in the sample is 52.6%.

Step-by-step explanation:

The percentage of glycine in the sample can be calculated by determining the amount of glycine reacted and dividing it by the total mass of the sample. Here are the steps to calculate the percentage:

1. Convert the pressure to atm: 735 torr / 760 torr/atm = 0.967 atm
2. Convert the temperature to Kelvin: 29 °C + 273 = 302 K
3. Use the ideal gas law to calculate the number of moles of N₂ gas: PV = nRT => n = PV/RT = (0.967 atm) * (0.00370 L) / (0.0821 L × atm/(mol × K)) * (302 K) = 4.237 × 10^-4 mol
4. Use the balanced equation to find the number of moles of glycine: 1 mol of glycine produces 1 mol of N₂, so the number of moles of glycine is also 4.237 × 10^-4 mol
5. Calculate the molar mass of glycine: (2 × 12.01 g/mol) + (5 × 1.008 g/mol) + (2 × 16.00 g/mol) + 14.01 g/mol = 75.04 g/mol
6. Calculate the mass of glycine in the sample: (4.237 × 10^-4 mol) * (75.04 g/mol) = 0.0318 g
7. Calculate the percentage of glycine: (0.0318 g / 0.0604 g) * 100% = 52.6%