When creating a calibration plot in volumetric analysis, how can you determine the concentration of Fe(SCN)2+(aq) in an unknown sample?

• A. Estimate the concentration based on the color intensity by eye.
• B. Calculate the concentration using the initial volumes of Fe3+ and SCN- added.
• C. Directly weigh the Fe(SCN)2+(aq) in the solution.
• D. Measure the absorbance of the unknown and use the calibration plot to find its concentration.

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the concept of a calibration plot. A calibration plot is a graph that relates the absorbance of a solution (measured using a spectrophotometer) to the concentration of the analyte. This is based on Beer-Lambert's law, which states that absorbance is directly proportional to concentration.
2. Step 2: Prepare a series of standard solutions with known concentrations of Fe(SCN)2+(aq). Measure the absorbance of each standard solution using a spectrophotometer at the wavelength where Fe(SCN)2+(aq) absorbs light most strongly.
3. Step 3: Plot the absorbance values of the standard solutions against their respective concentrations to create the calibration plot. Ensure the plot is linear, as Beer-Lambert's law applies within a linear range.
4. Step 4: Measure the absorbance of the unknown sample using the same spectrophotometer and wavelength used for the standard solutions.
5. Step 5: Use the calibration plot to determine the concentration of Fe(SCN)2+(aq) in the unknown sample. Locate the absorbance value of the unknown sample on the y-axis of the plot, then trace horizontally to the calibration line and vertically down to the x-axis to find the corresponding concentration.