Beers Law, maximum molar absorptivity of carbonyl molecule

In summary, the conversation discusses the calculation of the path length of a sample containing a carbonyl group in order to achieve specific levels of absorbance. The Beer's Law equation is used to calculate the path length, but the concentration of the sample is not given. The conversation also mentions the relationship between path length and absorbance, but it is not clear how this information helps solve the problem. The question about the meaning of "initial intensity" is raised, and it is suggested that it refers to the result of a previous experiment. The problem is ultimately asking how to adjust the path length to achieve specific levels of intensity for the passed radiation.
  • #1
Puchinita5
183
0
"Given that the maximum molar absorption coefficient of a molecule containing a carbonyl group is 30 dm3 mol-1 cm-1 near 280 nm, calculate the thickness of a sample that will result in (a) half the initial intensity of radiation, (b) one-tenth the initial intensity."

My attempt:

What is confusing me is I feel like I need the concentration to be given, but it's not.

A = -log(T) = εbc
where T is transmittance, ε is molar absorptivity, b is path length, and c is concentration.

I am understanding that the problem is looking for the path length, but I'm not completely sure about this.

I would do this for part A.

-log(0.5) = (30)*b*c
and solve for b.


how can i solve this? what am i missing?
 
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  • #2
What you need to understand is there is a relationship of path length (b) to absorption (A). Substitute 2b (twice the path length, right?) into your Beer's Law equation and see how that affects the absorbance. Now try 1/2b and 1/10b...
 
  • #3
Well yes, but I don't see how that helps me since I am still not given a concentration. I understand the general mathematical relationship of what happens, but I still feel like I am told to solve for one variable, but I have two unknowns. ? Perhaps I am not understanding what the problem is asking me.Obviously, if you increase the path length, you increase the absorbance. But not sure why this helps.
 
  • #4
If you are missing something, I am missing it as well. No problem with calculating bc, but I don't see how you can calculate just b.
 
  • #5
What do you think 'initial intensity' means? Do you think it means the intensity of the source or the result of the previous experiment? Only way this question can be answered is if you take it to mean the result of a previous experiment.

A sample of the carbonyl compound is measured in a cuvette of path length 'b'. The initial intensity of the passed radiation is measured by the photomultiplier. How do you reduce the initial intensity by changing path length so that the intensity of passed radiation is half as great? One tenth as great?
 

1. What is Beer's Law?

Beer's Law, also known as the Beer-Lambert Law, is a scientific principle that describes the relationship between the concentration of a solute in a solution and the amount of light absorbed by that solution. It states that the absorbance of a solution is directly proportional to the concentration of the solute and the path length of the light through the solution.

2. How is Beer's Law used in chemistry?

Beer's Law is used in chemistry to determine the concentration of a solute in a solution by measuring the absorbance of light passing through the solution. This can be done using a spectrophotometer, which measures the amount of light absorbed by a sample at a specific wavelength. By comparing the absorbance of a solution to a standard curve, the concentration of the solute can be calculated.

3. What is the maximum molar absorptivity of a carbonyl molecule?

The maximum molar absorptivity of a carbonyl molecule is the highest possible absorbance value that can be achieved by a solution containing a specific concentration of that molecule. It is a measure of how strongly a particular molecule absorbs light at a specific wavelength and is dependent on factors such as the structure and concentration of the molecule, as well as the solvent and path length of the light.

4. How does the maximum molar absorptivity of a carbonyl molecule differ from other molecules?

The maximum molar absorptivity of a carbonyl molecule can vary greatly from other molecules due to its unique structure and chemical properties. Carbonyl molecules, such as aldehydes and ketones, have a double bond between a carbon and an oxygen atom, which makes them highly reactive and capable of absorbing light at specific wavelengths. This differs from other molecules that may not have the same functional groups and therefore have different maximum molar absorptivity values.

5. What factors can affect the accuracy of using Beer's Law to determine the concentration of a carbonyl molecule?

Several factors can affect the accuracy of using Beer's Law to determine the concentration of a carbonyl molecule, including the purity and concentration of the sample, the path length of the light through the solution, and the presence of other substances that may interfere with the absorbance measurement. It is important to carefully control these variables in order to obtain accurate results when using Beer's Law in chemical analysis.

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