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. What was the percentage of glycine in the sample? CH2(NH2)CO2H + HNO2 ⟶ CH2(OH)CO2H + H2O + N2
To calculate the percentage of glycine in the sample, we need to determine the number of moles of N2 gas produced by the reaction.
Step 1: Calculate the partial pressure of N2 gas.
The pressure of N2 gas collected over water can be converted to the partial pressure by subtracting the vapor pressure of water at the given temperature:
Partial pressure of N2 gas = Total pressure - Vapor pressure of water
The vapor pressure of water at 29°C is 30 torr, so:
Partial pressure of N2 gas = 735 torr - 30 torr = 705 torr
Step 2: Convert partial pressure to moles.
We can use the ideal gas law to convert the partial pressure to moles of N2 gas:
PV = nRT
Where:
P = partial pressure (in atm)
V = volume of gas (in liters)
n = number of moles
R = ideal gas constant (0.0821 L·atm/(mol·K))
T = temperature (in Kelvin)
First, convert the partial pressure from torr to atm:
705 torr * (1 atm / 760 torr) = 0.927 atm
Next, convert the volume of the gas from milliliters to liters:
3.70 mL * (1 L / 1000 mL) = 0.00370 L
Now, convert the temperature from Celsius to Kelvin:
29 °C + 273.15 = 302.15 K
Plug the values into the ideal gas law equation to solve for moles:
(0.927 atm)(0.00370 L) = n(0.0821 L·atm/(mol·K))(302.15 K)
n = (0.927 atm * 0.00370 L) / (0.0821 L·atm/(mol·K) * 302.15 K)
n ≈ 0.0165 mol
Step 3: Calculate the molar mass of glycine.
The molar mass of glycine (CH2(NH2)COOH) can be calculated by adding up the atomic masses of each element:
C = 12.01 g/mol
H = 1.008 g/mol
N = 14.01 g/mol
O = 16.00 g/mol
Molar mass of glycine = (2 * 12.01 g/mol) + (5 * 1.008 g/mol) + 14.01 g/mol + 16.00 g/mol + 16.00 g/mol = 75.06 g/mol
Step 4: Calculate the mass of glycine in the sample.
The mass of glycine can be calculated using the moles of glycine and the molar mass:
Mass of glycine = moles of glycine * molar mass of glycine
Mass of glycine = 0.0165 mol * 75.06 g/mol
Mass of glycine ≈ 1.24 g
Step 5: Calculate the percentage of glycine in the sample.
The percentage of glycine can be calculated using the mass of glycine and the mass of the sample:
Percentage of glycine = (mass of glycine / mass of the sample) * 100
Percentage of glycine = (1.24 g / 0.0604 g) * 100
Percentage of glycine ≈ 2047% (rounded to three significant figures)
Therefore, the percentage of glycine in the sample is approximately 2047%.
To find the percentage of glycine in the sample, we need to determine the amount of glycine and divide it by the total mass of the sample.
First, let's calculate the number of moles of N2 gas produced using the Ideal Gas Law equation:
PV = nRT
Where:
P = pressure = 735 torr
V = volume of N2 = 3.70 mL = 0.00370 L
n = number of moles of N2 (what we want to find)
R = ideal gas constant = 0.0821 L·atm/(mol·K)
T = temperature = 29 °C = 29 + 273.15 K = 302.15 K
Simplifying the Ideal Gas Law equation and solving for n:
n = (PV) / (RT)
n = (735 torr * 0.00370 L) / (0.0821 L·atm/(mol·K) * 302.15 K)
Now, we need to convert the mass of glycine to moles:
Given mass of glycine = 0.0604 g
Molar mass of glycine = 75.07 g/mol
moles of glycine = (mass of glycine) / (molar mass of glycine)
moles of glycine = 0.0604 g / 75.07 g/mol
Finally, we can calculate the percentage of glycine in the sample:
% glycine = (moles of glycine / total moles) * 100
Now let's plug in the numbers and solve the equation to find the percentage of glycine in the sample.
Ptotal = pN2 + pH2O
735 mm = pN2 in mm + approx 31mm
Solve for pN2 in mm. Note: You need to look up the vapor pressure of water. I'm guessing at 31 mm at 29 C.
Then PV = nRT
You know P in mm. Convert that to atm.
You know V and R and T. Make T in Kelvin.
Solve for n = mols N2. There is 1 mol N2 evolved for each mole of glycine so mols N2 = mols glycine.
Then grams glycine = mols glycine x molar mass glycine.
Then % glycine = (grams glycine/grams sample)*100 = ?
Note: grams sample = 0.0604 g from the problem. Check my numbers.