Spectrophotometry example | Kinetics | Chemistry | Khan Academy
TLDRIn this educational video, the concept of spectrophotometry is explored through an example using potassium permanganate. The video demonstrates how to determine the concentration of a solute by measuring absorbance at a specific wavelength, applying the Beer-Lambert Law. Calibration data is plotted to show a linear relationship between absorbance and concentration, allowing for an estimation of the unknown concentration through visual inspection and further refinement using the proportionality constant. The process illustrates the practical application of the law in chemical analysis, providing a clear and engaging explanation.
Takeaways
- ๐ The script discusses a spectrophotometry example involving potassium permanganate, highlighting its absorbance properties at a specific wavelength.
- ๐ The absorbance of potassium permanganate at 540 nanometers is given as 0.539 in a 1 centimeter cell, indicating its sensitivity to solute concentration.
- ๐ Calibration data for the spectrophotometer is used to plot a graph of absorbance versus concentration, demonstrating the linear relationship predicted by the Beer-Lambert law.
- ๐ The Beer-Lambert law states that absorbance is directly proportional to the concentration of the solute and the path length of the light through the solution.
- ๐ The script involves plotting known concentrations of potassium permanganate and their corresponding absorbances to create a calibration curve.
- ๐งฉ By plotting the data points, a linear trend is observed, confirming the validity of the Beer-Lambert law for these measurements.
- ๐ The unknown concentration of potassium permanganate is determined by finding where its absorbance of 0.539 fits on the calibration curve.
- ๐ The script estimates the concentration by visually inspecting the calibration curve, suggesting it to be approximately 0.10 molar.
- ๐งฎ To find a more exact answer, the script calculates the proportionality constant (epsilon) using the Beer-Lambert law and one of the calibration data points.
- ๐ The calculated epsilon value is 5.40 liters per centimeter mole, which is used to determine the exact concentration of the unknown solution.
- ๐ The final concentration of the unknown potassium permanganate solution is calculated to be 0.0998 molar, closely matching the initial visual estimate.
Q & A
What is the main topic of the provided script?
-The main topic is determining the concentration of potassium permanganate in a solution using spectrophotometry and the Beer-Lambert law.
What wavelength is used to measure the absorbance of potassium permanganate?
-The wavelength used to measure the absorbance is 540 nanometers.
What is the absorbance of the potassium permanganate solution at 540 nanometers?
-The absorbance of the potassium permanganate solution at 540 nanometers is 0.539.
What is the length of the cell used for the spectrophotometry measurements?
-The length of the cell used for the measurements is 1 centimeter.
What law is used to determine the concentration of the potassium permanganate solution?
-The Beer-Lambert law is used to determine the concentration of the potassium permanganate solution.
What is the relationship between absorbance and concentration according to the Beer-Lambert law?
-According to the Beer-Lambert law, there is a linear relationship between absorbance and concentration.
How is the concentration of the unknown potassium permanganate solution determined in the script?
-The concentration is determined by plotting the calibration data on a graph, finding the linear relationship, and locating the absorbance value of the unknown solution on the graph to read off the concentration.
What is the calculated proportionality constant (epsilon) for potassium permanganate at 540 nanometers?
-The calculated proportionality constant (epsilon) for potassium permanganate at 540 nanometers is 5.40 liters per centimeter mole.
How is the exact concentration of the unknown potassium permanganate solution calculated using the Beer-Lambert law?
-The exact concentration is calculated by dividing the absorbance (0.539) by the product of the proportionality constant (5.40) and the cell length (1 cm), resulting in a concentration of 0.0998 molarity.
How does the estimated concentration from the graph compare to the exact concentration calculated using the Beer-Lambert law?
-The estimated concentration from the graph is approximately 0.10 molarity, which is very close to the exact concentration of 0.0998 molarity calculated using the Beer-Lambert law.
What is the purpose of plotting the calibration data on a graph?
-The purpose of plotting the calibration data on a graph is to establish the linear relationship between absorbance and concentration, which allows for the determination of the unknown concentration by locating its absorbance value on the graph.
Outlines
๐ฌ Introduction to Spectrophotometry Problem
The video script introduces a spectrophotometry problem involving potassium permanganate. It discusses the absorbance of a solution measured at a specific wavelength, 540 nanometers, using a 1 cm cell. The goal is to determine the concentration of potassium permanganate in the solution. The script mentions calibration data that will be plotted to establish a linear relationship between absorbance and concentration, based on the Beer-Lambert law.
๐ Plotting Calibration Data and Applying the Beer-Lambert Law
The script proceeds to plot the calibration data, which includes known concentrations of potassium permanganate and their corresponding absorbance values. It visually demonstrates the linear relationship predicted by the Beer-Lambert law, which states that absorbance is directly proportional to the concentration of the solute. The video then uses this relationship to estimate the concentration of an unknown solution with an absorbance of 0.539 by locating it on the plotted line.
๐ Calculating the Proportionality Constant and Determining Concentration
The script explains how to calculate the proportionality constant (epsilon) using the Beer-Lambert law. It selects a data point from the calibration plot to determine this constant, which is found to be 5.40 liters per centimeter mole. Using this constant, the script then calculates the exact concentration of the unknown solution with an absorbance of 0.539, resulting in an approximate molarity of 0.0998 M. This calculated value closely matches the earlier estimation made by visual inspection of the plot.
Mindmap
Keywords
๐กSpectrophotometry
๐กPotassium Permanganate
๐กAbsorbance
๐กWavelength
๐กBeer-Lambert Law
๐กCalibration Data
๐กMolarity
๐กConcentration
๐กProportionality Constant
๐ก1 Centimeter Cell
๐กLinear Relationship
Highlights
The problem involves determining the concentration of potassium permanganate using spectrophotometry.
The absorbance of potassium permanganate is measured at 540 nanometers, a wavelength it absorbs well.
The measurement is taken in a 1 centimeter cell, which affects the absorbance reading.
Calibration data for the spectrophotometer is collected using known concentrations of potassium permanganate.
A plot of absorbance against concentration is used to determine the unknown concentration.
The Beer-Lambert law states that absorbance is linearly related to concentration.
The absorbance of 0.539 for the unknown solution is plotted on the calibration graph.
The concentration of the unknown solution is estimated by its position on the calibration graph.
The Beer-Lambert law is used to calculate the exact concentration using the absorbance and a proportionality constant.
The proportionality constant is calculated using the first data point from the calibration graph.
The calculated proportionality constant is 5.40 liters per centimeter mole.
The exact concentration of the unknown solution is calculated to be 0.0998 moles per liter.
The estimated concentration from the graph is close to the exact calculation, showing the accuracy of the method.
The method demonstrates the practical application of the Beer-Lambert law in spectrophotometry.
The process involves plotting data points, drawing a line of best fit, and reading off the concentration from the graph.
The absorbance values and concentrations are used to validate the linear relationship predicted by the Beer-Lambert law.
The proportionality constant is crucial for accurately determining the concentration from absorbance measurements.
The example illustrates the steps involved in using spectrophotometry to analyze chemical concentrations.
Transcripts
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