Solve Beer's Law: Find Phenolphthalein Concentration from 0.80 Absorbance Unit

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Discussion Overview

The discussion revolves around solving a homework problem related to Beer's Law, specifically determining the concentration of phenolphthalein from a measured absorbance of 0.80 at 560nm. The conversation includes considerations of necessary parameters such as path length and molar absorptivity, as well as the implications of missing information.

Discussion Character

  • Homework-related, Technical explanation, Debate/contested

Main Points Raised

  • One participant notes the need to solve for concentration (c) using Beer's Law, indicating uncertainty about the required information.
  • Another participant clarifies that the path length (b) is typically 1 cm, but acknowledges that molar absorptivity (e) can vary with conditions such as temperature and pH, suggesting an estimated value for phenolphthalein.
  • A later reply expresses concern about the lack of sufficient information to calculate an answer based solely on the question.
  • There is a discussion about the terminology used to describe the cuvette length, with differing opinions on whether calling it a "convention" is appropriate.

Areas of Agreement / Disagreement

Participants generally agree that the cuvette length is commonly 1 cm, but there is disagreement on the appropriateness of referring to this as a "convention." Additionally, there is no consensus on whether the problem can be solved with the given information.

Contextual Notes

The discussion highlights limitations such as the unspecified cuvette dimensions and the variability of molar absorptivity, which may affect the ability to solve the problem accurately.

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Homework Statement


If I measure an absorbance at 560nm of 0.80 absorbance units for a solution of the basic form of phenolphthalein, what is the concentration of phenolphthalein?

Homework Equations


Beer's Law -- A(sub lamda)=e*b*c

The Attempt at a Solution



The question is asking for concentration, so I'll need to solve for c. I think A is the absorbance units, but beyond that I believe I'm missing information.

b is supposed to be the path length, which I believe is the width of whatever cuvet is being used (the problem doesn't specify what that is). e is molar absorbtivity, but isn't that a constant?

Is this problem able to be solved as it is or do I need more information?

Thanks!
 
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TrueStar said:
b is supposed to be the path length, which I believe is the width of whatever cuvet is being used (the problem doesn't specify what that is). e is molar absorbtivity, but isn't that a constant?

Cuvet length is 1 cm by convention. Molar absorptivity is going to vary with temperature and pH as you can see here:

http://pubs.acs.org/doi/full/10.1021/ol016823q

Unfortunately they don't report a value for phenolphthalein but estimating from Fig. 6, e should be about 1770/(M.cm) at 20C and basic conditions.
 
Thank you for the reply. It seems there was no way to calculate an answer based on the question alone. I was worried I was missing something obvious.

I will ask my lab instructor about it next week.
 
presbyope said:
Cuvet length is 1 cm by convention.

I agree 1 cm is a common cuvette size, but calling it a "convention" is too far fetched.
 
Last edited by a moderator:
Borek said:
I agree 1 cm is a common cuvette size, but calling it a "convention" is too far fetched.

Quite right, I stand corrected.
 

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