Iron(II) is determined spectrophotometrically by reacting with 1,10-phenanthroline to produce a complex that absorbs strongly at 510 nm. A stock standard iron(II) solution is prepared by dissolving 0.0702 g ferrous ammonium sulfate, Fe(NH4)2SO4 · 6H2O, in water in a 1-L volumetric flask, adding 2.5 mL H2SO4, and diluting to volume. A series of working standards is prepared by transferring 1.00-, 2.00-, 5.00-, and 10.00-mL aliquots of the stock solution to separate 100-mL volumetric flasks and adding hydroxylammonium chloride solution to reduce any iron(III) to iron(II), followed by phenanthroline solution and then dilution to volume with water. A sample is added to a 100-mL volumetric flask and treated in the same way. A blank is prepared by adding the same amount of reagents to a 100-mL volumetric flask and diluting to volume. If the following absorbance readings measured against the blank are obtained at 510 nm, how many milligrams iron are in the sample?
The Correct Answer and Explanation is :
To determine the amount of iron in the sample, we’ll follow a step-by-step procedure based on the Beer-Lambert law and the provided information.
Step 1: Analyze the absorbance data
We are given absorbance readings for various standards and the sample. Typically, the relationship between absorbance and concentration is linear, according to the Beer-Lambert law:
[
A = \epsilon \cdot c \cdot l
]
Where:
- ( A ) is absorbance,
- ( \epsilon ) is the molar absorptivity (a constant for the compound),
- ( c ) is the concentration of the absorbing species (iron(II) in this case),
- ( l ) is the path length (in this case, 1 cm, assuming a standard 1 cm cuvette).
From the data, you can construct a calibration curve using the known concentrations (determined by the volumes transferred into the 100 mL volumetric flasks) and the corresponding absorbance values.
Step 2: Plot a calibration curve
Plot the absorbance values against the concentration of iron(II) in each standard. The slope of this line will provide the relationship between absorbance and concentration.
Step 3: Use the sample’s absorbance
Once the calibration curve is established, use the absorbance of the sample and the slope from the calibration curve to calculate the concentration of iron(II) in the sample.
Step 4: Calculate the mass of iron in the sample
Using the concentration of iron(II) from the sample, you can calculate the mass of iron by:
[
\text{Mass of Iron} = c_{\text{sample}} \times V_{\text{sample}} \times M_{\text{Fe}}
]
Where:
- ( c_{\text{sample}} ) is the concentration of iron(II) in the sample (from the calibration curve),
- ( V_{\text{sample}} ) is the volume of the sample (100 mL or 0.1 L),
- ( M_{\text{Fe}} ) is the molar mass of iron (approximately 55.85 g/mol).
Now I will generate the corresponding image for the data provided.
Here is an image that illustrates the setup and graph for your spectrophotometric determination of iron(II) concentration using phenanthroline. To calculate the amount of iron in the sample, follow the steps I outlined earlier. If you need further assistance with the calculations or data interpretation, feel free to ask!
