Table of contents

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

16. Conjugated Systems

Conjugation Chemistry

Organic Chemistry

16. Conjugated Systems

Conjugation Chemistry

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7:07 minutes

Problem 21

Textbook Question

One milligram of a compound of molecular weight 160 is dissolved in 10 mL of ethanol, and the solution is poured into a 1-cm UV cell. The UV spectrum is taken, and there is an absorption at λ_max = 247 nm. The maximum absorbance at 247 nm is 0.50. Calculate the value of e for this absorption.

Verified step by step guidance

Step 1: Recall the Beer-Lambert Law, which is expressed as:

A = ε × c × l

, where A is the absorbance, ε is the molar absorptivity (in L·mol⁻¹·cm⁻¹), c is the concentration (in mol·L⁻¹), and l is the path length of the cell (in cm).

Step 2: Identify the given values from the problem: A = 0.50, l = 1 cm, and the molecular weight (MW) of the compound is 160 g/mol. The mass of the compound dissolved is 1 mg (0.001 g), and the volume of the solution is 10 mL (0.01 L).

Step 3: Calculate the concentration (c) of the solution in mol·L⁻¹. Use the formula:

c = rac{m}{MW × V}

, where m is the mass of the compound, MW is the molecular weight, and V is the volume of the solution in liters.

Step 4: Rearrange the Beer-Lambert Law to solve for ε (molar absorptivity):

ε = rac{A}{c × l}

. Substitute the values of A, c, and l into this equation.

Step 5: Perform the calculation to determine the value of ε. Ensure the units are consistent throughout the calculation, and the final value of ε will be in L·mol⁻¹·cm⁻¹.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Beer-Lambert Law

The Beer-Lambert Law relates the absorbance of light to the properties of the material through which the light is traveling. It states that absorbance (A) is directly proportional to the concentration (c) of the absorbing species, the path length (l) of the light through the sample, and the molar absorptivity (ε) of the substance. The equation is A = εcl, which is essential for calculating the molar absorptivity in this context.

Molar Absorptivity (ε)

Molar absorptivity, or molar extinction coefficient (ε), is a measure of how strongly a chemical species absorbs light at a given wavelength. It is expressed in units of L/(mol·cm) and is specific to each substance at a particular wavelength. In this problem, calculating ε will provide insight into the compound's ability to absorb UV light at 247 nm.

Concentration Calculation

To apply the Beer-Lambert Law, it is necessary to determine the concentration of the compound in the solution. Concentration (c) can be calculated using the formula c = m/MV, where m is the mass of the solute, M is the molar mass, and V is the volume of the solution in liters. In this case, the concentration will be crucial for finding the molar absorptivity.

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(a)

(b)

(c)

Textbook Question

Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous sp² hybridized atoms.

(d)

(e)

(f)

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A 4.0 × 10^-5 M solution of a compound in hexane shows an absorbance of 0.40 at 252 nm in a cell with a 1 cm light path. What is the molar absorptivity of the compound in hexane at 252 nm?

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What would be the difference in the colors of the compounds at pH = 3?

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