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1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

17. Ultraviolet Spectroscopy

The Beer-Lambert Law

17. Ultraviolet Spectroscopy

The Beer-Lambert Law: Videos & Practice Problems

Video Lessons Practice Worksheet

Topic summary

The Beer-Lambert law describes the linear relationship between a solution's concentration and its light absorbance. The absorbance (A) is calculated using the equation: A=log(I₀), where I₀ is the initial light intensity and I is the transmitted intensity. This relationship also incorporates molar absorptivity (ε), concentration (c), and path length (l): A=ε·c·l. Understanding this law is crucial for quantitative analysis in absorption spectroscopy.

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The Beer-Lambert Law Concept 1

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The Beer-Lambert Law Concept 1 Video Summary

The Beer-Lambert Law describes a fundamental relationship in analytical chemistry, illustrating how the concentration of a solution affects its ability to absorb light. This law is expressed mathematically as:

A = εlc

In this equation, A represents the absorbance of the solution, ε is the molar absorptivity (a constant that indicates how strongly a substance absorbs light at a particular wavelength), l is the path length of the light through the solution (typically measured in centimeters), and c is the concentration of the solution (measured in moles per liter).

When light passes through a solution, the intensity of the light before entering the solution is denoted as I₀, while the intensity after passing through the solution is I. According to the Beer-Lambert Law, the relationship between these intensities can be expressed as:

A = log₁₀(I₀/I)

This indicates that as the concentration of the solution increases, the absorbance A also increases, leading to a decrease in the transmitted light intensity I. Consequently, a more concentrated solution will absorb more light, resulting in a lower value of I compared to I₀.

Graphically, this relationship can be represented with absorbance plotted on the y-axis and concentration on the x-axis, typically resulting in a linear graph. The peak of this graph, known as the lambda max, indicates the wavelength at which the solution absorbs the most light. Understanding this relationship is crucial for accurately determining the concentration of unknown solutions through spectrophotometric methods.

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The Beer-Lambert Law Concept 2

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The Beer-Lambert Law Concept 2 Video Summary

The Beer-Lambert Law is a fundamental principle in chemistry that describes the relationship between absorbance and the properties of a sample. The equation can be expressed as:

$ A = \log\left(\frac{I_0}{I}\right) = \varepsilon \cdot c \cdot l $

In this equation, $ A $ represents the absorbance of the sample, which is a measure of how much light is absorbed by the sample. The term $ I_0 $ denotes the intensity of the incident light, while $ I $ is the intensity of light that passes through the sample. The absorbance can be calculated by taking the logarithm of the ratio of these two intensities.

The equation also highlights the role of molar absorptivity ($ \varepsilon $), concentration ($ c $), and path length ($ l $). Molar absorptivity is a constant that indicates how strongly a substance absorbs light at a particular wavelength, measured in units of liters per mole per centimeter ($ \text{L} \cdot \text{mol}^{-1} \cdot \text{cm}^{-1} $). The concentration $ c $ is typically expressed in molarity (M), which is moles of solute per liter of solution. The path length $ l $ refers to the distance the light travels through the sample, measured in centimeters.

Understanding this equation is crucial for performing calculations related to the Beer-Lambert Law. By recognizing that absorbance can be interpreted in two ways—either through the logarithmic relationship of light intensities or as a product of molar absorptivity, concentration, and path length—students can effectively apply this law in various analytical chemistry contexts.

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The Beer-Lambert Law Example 1

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The Beer-Lambert Law Example 1 Video Summary

In this example, we are tasked with calculating the ratio of the initial intensity (I₀) to the transmitted intensity (I) based on the absorbance of a solution. The relationship between absorbance (A) and the intensity ratio is defined by the formula:

A = log₁₀(I₀/I)

Given that the absorbance is 0.602, we can substitute this value into the equation:

0.602 = log₁₀(I₀/I)

To find the ratio I₀/I, we need to eliminate the logarithm by taking the inverse log of both sides. This is done by raising 10 to the power of both sides:

10^0.602 = I₀/I

Calculating 10^0.602 using a calculator yields approximately 3.999, which we can round to 4. Therefore, the ratio I₀/I is 4, indicating that for every 4 units of initial intensity, there is 1 unit of transmitted intensity. This can be expressed as a 4:1 ratio.

In summary, the final answer is a 4:1 ratio, meaning that the initial intensity is four times greater than the transmitted intensity.

Problem

A 1.6 x 10^-3 g sample of compound (MM = 136 g/mol) was dissolved in 15 mL of methanol. Maximum absorption at λ _max (258 nm) represents absorbance of 0.73. Determine the molar absorptivity of the sample if the light travels through a 1.6 cm UV cell.

581718 M^-1 cm^-1

58.0 M^-1 cm^-1

2.06 x 10⁵ M^-1 cm^-1

580 M^-1 cm^-1

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What is the Beer-Lambert Law and how is it used in chemistry?

The Beer-Lambert Law describes the linear relationship between the concentration of a solution and its absorbance of light. It is used in chemistry to determine the concentration of a solute in a solution by measuring the absorbance of light at a specific wavelength. The law is expressed by the equation:

$A = ε \cdot c \cdot l$

where $A$ is the absorbance, $ε$ is the molar absorptivity, $c$ is the concentration, and $l$ is the path length. This law is crucial for quantitative analysis in absorption spectroscopy.

How do you calculate absorbance using the Beer-Lambert Law?

To calculate absorbance using the Beer-Lambert Law, you use the equation:

$A = ε \cdot c \cdot l$

where $A$ is the absorbance, $ε$ is the molar absorptivity (in L·mol^-1·cm^-1), $c$ is the concentration of the solution (in mol/L), and $l$ is the path length of the light through the solution (in cm). By knowing the values of $ε$ , $c$ , and $l$ , you can calculate the absorbance $A$ .

What factors affect the absorbance in the Beer-Lambert Law?

The absorbance in the Beer-Lambert Law is affected by three main factors:

Concentration (c): Higher concentration of the absorbing species increases absorbance.
Path Length (l): Longer path length through which light travels increases absorbance.
Molar Absorptivity (ε): This is a constant that depends on the nature of the absorbing species and the wavelength of light used. Higher molar absorptivity means higher absorbance for a given concentration and path length.

These factors are incorporated into the Beer-Lambert Law equation:

$A = ε \cdot c \cdot l$

What is molar absorptivity and how is it determined?

Molar absorptivity, denoted as $ε$ , is a measure of how strongly a chemical species absorbs light at a given wavelength. It is determined experimentally by measuring the absorbance of a series of known concentrations of the species and plotting absorbance (A) against concentration (c). The slope of the resulting linear plot, when the path length (l) is known, gives the molar absorptivity:

$ε = \frac{A}{c \cdot l}$

Molar absorptivity has units of L·mol^-1·cm^-1 and is specific to the substance and the wavelength of light used.

How does the Beer-Lambert Law apply to real-world applications?

The Beer-Lambert Law is widely used in various real-world applications, particularly in analytical chemistry and biochemistry. It is used to determine the concentration of substances in solutions, such as measuring the concentration of pollutants in water, determining the concentration of DNA or proteins in biological samples, and monitoring the progress of chemical reactions. By measuring the absorbance of light at specific wavelengths, scientists can quantify the amount of a substance present in a sample, making it a crucial tool for quality control, environmental monitoring, and research.