PRELAB GUIDING QUESTIONS/SIMULATIONS Step 1: You will be collecting data (in you

Question

PRELAB GUIDING QUESTIONS/SIMULATIONS Step 1: You will be collecting data (in your group) to determine the relationship between wavelength (lambda, ), concentration, and absorbance. You will use the interaction of light with solutions to determine the concentration of an absorbing species. However, it is important to identify what is doing the absorbing prior to determining the concentration of the absorbing species. Such information can be gathered by collecting absorbance data for a solution at various wavelengths. Step 2: Your group will be given two different salt solutions of the exact same concentration. You are to measure the absorbance for each solution at every 20 nanometers from 400-700 nm to generate a spectrum and to determine the best wavelength at which to measure the absorbance of the two solutions. Make sure you use the appropriate spectrophotometer or colorimeter to collect such data. You will be sharing your absorption spectra with the rest of the class to identify what species in solution is actually absorbing light.

Explanation / Answer

Extension question 1 :

Mostly all the compounds show their maximum absorbance in the range 400-700 nm, so it is important to make an absorption vs wavelength plot for this range to find out the maximum absorbance.The need for choosing a particular wavelength(of maximum absorbance) is that it gives a high signal to noise ratio. This means that if we are interested in determining the extent of absorption by a particular molecule amongst a mixture of 5 molecules, we should check at the wavelength at which absorption for that particular molecule is maximum , as this would reduce the interference in reading produced by the presence of other molecules.

Extension question 2 :

From the Beer Lambert's law, we have :

A = e*l*c

Here e is molar absorptivity

Keeping everything else constant, we see that A is directly proportional to e.

Thus. If e reduces by half, A also reduces by half.

So, from the absorption vs wavelength plot, we simply go to the point where A is half of the initial value, because that will also indicate that e is also half of the original value. The wavelength corresponding to that particular point where A is half should be used.

Related Questions

Navigate

• Browse (All)
• Browse P
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.