Question

The following questions are from a copper content in brass experiment in my Chem 1 lab class. These are the discussion questions, I would like a little bit of background knowledge on how I may get the answers so I could fully understand. Thank you!

The equation for your line has a nonzero y intercept. Beer’s law does not have an analogous term. Explain the discrepancy between your experimental best fit equation and

the theoretical Beer’s law equation.

2. Consider the experimental procedure. What were the most likely sources of error?

Remember that experimental error is not the same thing as a calculation mistake or incorrectly following the procedure.

3. In a previous experiment we used the λ max to determine the absorbances to use for the

calibration graph. Why did we other criteria (not λmax) in this experiment? Is Beer’s law valid at other wavelengths? Explain.

4. Based the absorbance vs. wavelength spectrum of the dissolved brass solution, is there evidence of the presence of elements other than copper and zinc in the brass sample?

Explain your answer.

Answer 1

Pre Requisite:

Beer-Lambert's Law

• Therefore the absorption at two different wavelength will not be same as the molar absorption coefficient is different for different wavelength.
• Also, the absorption values for two different samples are not same. Which answeres your Question No. 4 as if any other element is present, it will show Absorption Peaks at different wavelengths.

Beer-Lambert's Law is highly sensitive to concentration as the Beer-Lambert Plot doesnt follow linearity when the high concentrations are taken into account.

Answer 3: As I showed above that the _max is different for different wavelength and hence we must stick to One single wavelength for whole experiment. and we can do the same experiment with different wavelegth also but that wavelength value must be used for all measurements and also it must not be interfered with other elements or impurities peak so that the calculation error does not occur.

As the Brass contains Zinc and Copper mainly, it's called as alloy of Copper and is available commercially in many forms where the percentage of Copper and Zinc are altered and its determination is necessary in order to identify or check for the Product quality.
So in this experiment, the Brass is dissolved in Nitric Acid so as to dissolve the the solid cooper to make Copper(II) Solution.
This part must be done carefully as Nitric Acid is used here and dissolving Brass may spill the acid and also the NO,NO₂ are evolved which are very toxic and must be handled carefully.

This equation is followed during dissolving process and the Cu²⁺ is formed. as you asked for getting the background knowledge, Suppose you want to know the concentration of one unknown sample of Cu²⁺, Then what you should do is that you should make 3-4 known concentrations of Cu²⁺ Samples and then make a Concentration v/s absorbance curve(remember to keep only one value for all samples) and then measure the absorbance of unknown conc sample. and then plot the absorbance value of unknown in the previous Conc V/s Absorbance Curve.
By this method you can easily find out the concentration of unknown sample.

The same method is followed here in this case.

Possible cause of errors
1. Low concentrations of sample must be used as beer lambert law fails at high concentration.

2. UV-Spectrophotometer must be properly calibrated before use, incorrectly calibrated spectrophotometer causes errors in data.

3. Deionised Water must be used for experiment as Tap water already contains some Cu²⁺, which will cause inaccuracy.

Hope I answered