# Absorbance and Stray light helppp

• atomicblast
In summary, the conversation is about a chemistry assignment involving the calculation of stray light, Ps, as a percentage of the incident light, Po. The given information includes the apparent absorbance, concentration, cell length, and activity coefficients. The transmittance is calculated using the equation A = -log T, where T = 0.8974. However, there are still two unknowns in the equation and the person is seeking advice on how to proceed. The conversation also mentions the question given for the assignment, which involves measuring the apparent absorbance of a solution of a weak acid, HA, at different pH values. The results for the absorbance are provided and the person is asked to calculate the stray light as a percentage of the
atomicblast
Hi, I've a chem assignment in which I've to calculate the stray light, Ps as a percentage of the incident light, Po.

Given the apparent absorbance A(app) 0.047, concentration of the solution 2.00 x 10pwr-6, cell length of 1cm, activity coefficients = 1.

I calculated the transmittance to be A = -log T , where T = 0.8974

T = P + Ps / Po + Ps = 0.8974
T = P / Po = 0.8974

I'm stuck after that with 2 unknowns in the equation. Could someone advice me on this?

Thanks a lot!

It would help (yourself) a great deal if you could describe the experiment involved.

Okie, this is the question given:

The apparent absorbance of a solution of a weak acid, HA, was measured at different pH values (you may assume the pH meter was correctly calibrated, that activity coefficients are unity throughout, and that concentrations are reliable. A 1.00cm cell was used. None of the buffers used absorbs at the wavelength used. Results were as follows:

Concentration, M - 2.00 x 10pwr-6

Apparent Absorbance:
0.047 at pH 2,
0.045 at pH 4,
0.018 at pH 6,
0.010 at pH 8,
0.009 at pH 10

1) Calculate the stray light, Ps, as a percentage of the incident light, Po, in the spectrophotometer used.

Thanks again!

## 1. What is absorbance?

Absorbance is a measure of how much light is absorbed by a substance at a specific wavelength. It is represented by the symbol "A" and is calculated using the formula A = log(Io/I), where Io is the initial incident light intensity and I is the transmitted light intensity.

## 2. How is absorbance related to concentration?

Absorbance is directly proportional to the concentration of a substance in a solution. This means that as the concentration increases, the absorbance also increases. This relationship is known as Beer's Law and is commonly used in spectrophotometry to determine the concentration of a substance in a solution.

## 3. What is stray light and how does it affect absorbance readings?

Stray light is unwanted light that enters the spectrophotometer and affects the accuracy of absorbance readings. It can come from various sources such as reflections, scattering, and imperfect filters. Stray light can cause errors in absorbance readings and should be minimized or corrected for in experiments.

## 4. How can stray light be reduced?

Stray light can be reduced by using quality optical components, such as filters and lenses, that minimize reflections and scattering. Additionally, proper instrument maintenance, such as cleaning the cuvettes and checking for any light leaks, can also help reduce stray light. Some spectrophotometers also have software features that can correct for stray light.

## 5. Can stray light affect the accuracy of absorbance measurements at all wavelengths?

Yes, stray light can affect the accuracy of absorbance measurements at all wavelengths, although it is more pronounced at longer wavelengths. This is because stray light is typically composed of longer wavelengths that overlap with the wavelengths being measured, leading to an overestimation of the absorbance value. It is important to calibrate and correct for stray light at all wavelengths to obtain accurate results.

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