Spectrophotometric Determination of Fe2+ (Beer's Law)

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SUMMARY

The discussion focuses on calculating the percent by mass of iron (Fe2+) in an unknown sample using Beer's Law. The slope of the Beer’s Law plot is 0.2200 L/mg Fe2+ and the intercept is 0.004. Given a sample mass of 0.1097g and an absorbance of 0.252, the user struggles with determining the correct concentration and volume to use for calculating moles and grams of iron. The correct approach involves understanding the dilution factor from the initial solution to the final spectroscopic measurement.

PREREQUISITES
  • Understanding of Beer's Law and its application in spectrophotometry
  • Knowledge of concentration calculations and dilution principles
  • Familiarity with absorbance and its relation to concentration
  • Basic skills in converting mass to moles using molar mass
NEXT STEPS
  • Calculate concentration using the formula: Concentration = (Absorbance - Intercept) / Slope
  • Learn about dilution factors and how they affect concentration calculations
  • Study the relationship between initial and final concentrations in serial dilutions
  • Explore the molar mass of iron to convert moles to grams accurately
USEFUL FOR

Chemistry students, laboratory technicians, and anyone involved in quantitative analysis using spectrophotometry.

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Homework Statement
I need to determine the % by mass of iron for an experiment
Relevant Equations
y=mx+b, and stoich
I am having troubles with a question I have to do for my chem pre-lab. The question is:
The following intercept and slope were obtained for a Beer’s Law plot of Absorbance versus concentration (mg Fe2+/L).
Slope = 0.2200 L/mg Fe2+
Intercept = 0.004
Calculate the percent (by mass) of iron in the unknown sample given the following mass and absorbance, assume the experimental data is obtained using the same procedure as outlined in the experiment.
Mass sample = 0.1097g
Absorbance = 0.252
I have attached what the procedure is. I kind of understand how to get the concentration using the intercept, slope, and absorbance. But, once I get the concentration I don't know what volume to use to calculate the moles to get to the amount of grams. I had used 0.01L for my volume and did not get the correct answer. Also, I tried to use the different concentrations and volumes from step 8 through 12 but still did not get the correct answer. If someone could explain this it would be very much appreciated
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You made up a solution containing the given mass in 100 mL. You then diluted this twice, and used the last solution for spectroscopy.
What is the relation of the concentration in the last solution to that in the first?
What concentration did you obtain from the spectroscopic measurement?
What is the concentration of the initial solution?
How much iron is in this solution?
 

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