A standard addition method is used to determine the concentration of ethanol in an
unknown sample. Six 10 mL aliquots of the unknown are drawn from the processing
vat. An aqueous solution of ethanol is standardized to have 1.57 g/L. This standard is
pipetted to five of the aliquots in the following volumes: 5.00 mL, 10.00 mL, 15.00 mL,
20.00 mL, and 25.00 mL. Each of the six solutions is then diluted to a total volume of
50.00 mL.
Is there a question here? Is there any absorbance data here? I don't see either.
To determine the concentration of ethanol in the unknown sample using the standard addition method, here are the steps:
Step 1: Calculate the moles of ethanol in the standardized solution.
First, convert the concentration of the standardized solution from grams per liter (g/L) to moles per liter (mol/L). The molar mass of ethanol (C2H6O) is 46.07 g/mol.
So, 1.57 g/L is equal to (1.57 g/L) / (46.07 g/mol) = 0.034 mol/L.
Step 2: Determine the moles of ethanol added to each aliquot.
Since the aliquots have different volumes, we need to calculate the moles of ethanol added to each one.
For the 5.00 mL aliquot:
Moles of ethanol = (0.034 mol/L) × (5.00 mL/1000 mL) = 0.00017 mol
For the 10.00 mL aliquot:
Moles of ethanol = (0.034 mol/L) × (10.00 mL/1000 mL) = 0.00034 mol
For the 15.00 mL aliquot:
Moles of ethanol = (0.034 mol/L) × (15.00 mL/1000 mL) = 0.00051 mol
For the 20.00 mL aliquot:
Moles of ethanol = (0.034 mol/L) × (20.00 mL/1000 mL) = 0.00068 mol
For the 25.00 mL aliquot:
Moles of ethanol = (0.034 mol/L) × (25.00 mL/1000 mL) = 0.00085 mol
Step 3: Calculate the total moles of ethanol in each diluted solution.
Since each aliquot is diluted to a total volume of 50.00 mL, we can use the dilution formula to calculate the concentration:
Moles of ethanol in diluted solution = Moles of ethanol added / Total volume of solution
For each aliquot, the total volume is 50.00 mL, so the calculation becomes:
For the 5.00 mL aliquot:
Moles of ethanol in diluted solution = 0.00017 mol / (50.00 mL/1000 mL) = 0.0034 mol/L
For the 10.00 mL aliquot:
Moles of ethanol in diluted solution = 0.00034 mol / (50.00 mL/1000 mL) = 0.0068 mol/L
For the 15.00 mL aliquot:
Moles of ethanol in diluted solution = 0.00051 mol / (50.00 mL/1000 mL) = 0.0102 mol/L
For the 20.00 mL aliquot:
Moles of ethanol in diluted solution = 0.00068 mol / (50.00 mL/1000 mL) = 0.0136 mol/L
For the 25.00 mL aliquot:
Moles of ethanol in diluted solution = 0.00085 mol / (50.00 mL/1000 mL) = 0.017 mol/L
Step 4: Determine the concentration of ethanol in the unknown sample.
Since one of the aliquots is without any added standard, we can assume that the concentration of ethanol in the unknown sample is equal to the moles of ethanol in the unknown aliquot (before any addition) divided by the total volume of the diluted unknown solution.
For the unknown aliquot (without any added standard):
Moles of ethanol in diluted solution = 0.034 mol/L (since no addition made)
Total volume of the diluted unknown solution = 50.00 mL
Concentration of ethanol in the unknown sample = 0.034 mol/L / (50.00 mL/1000 mL) = 0.68 mol/L
Therefore, the concentration of ethanol in the unknown sample is 0.68 mol/L.