[dan111man]

Alright, so these are some prelab questions I have to answer by 17:00 ET on 1/20/11. PLEASE do not answer the questions for me, just point me in the right direction; I would like to do the rest for the sake of memorization. Anything you have to offer is greatly appreciated.

A student was given a stock aluminum(III) solution with a concentration of 5.000 parts per million (ppm). (ppm are defined as mg/L for dilute aqueous solutions.) The student prepared five 100-mL standard solutions and an unknown as described in the procedure section of the experiment. The absorbance of each solution was read at 565 nm, using the black to set zero absorbance. The results are tabulated below.
                mL of Al(III) stock
Solution         Solution Used         Absorbance
_______       _____________        __________

1                        8.00                    0.541
2                        6.00                    0.424
3                        4.00                    0.294
4                        2.00                    0.163
5                        1.00                    0.098
Unknown              ----                    0.388


1. For each of the five standard solutions calculate the concentration of Al(III) in ppm. (Hint: this is a dilution, you do not need to use Beer's Law to determine the concentrations.)

2. Use graphical analysis or a sheet of graph paper to prepare a plot of absorbance (y) vs. concentration (x). Based upon your curve, was the blank too dark or too light?

3. What is your best estimate of the AL(III) concentration of the unknown sample in the cuvet based upon where its absorbance falls on the standard curve?

4. Use your answer to question 3 to calculate the Al(III) concentration in the original unknown solution by correcting for the dilution made when preparing the sample for analysis.

Thank you very much!!!

[dan111man]

I just don't understand how to relate the mLs and absorbance values to that of concentration. I've tried it a few times based off equations I've found online, but I keep getting numbers in the tens and I don't think that can be right. Also, I just realized how to do the graphical analysis so that is no longer a problem. As far as the estimate goes, do I just calculate the value of a point in the middle of my standard curve?

Thank you for your time and consideration.

[tamim83]

Dilution formula: C₁V₁=C₂V₂. 
C= Concentration (in ppm for your problem) and V=Volume. 

Hope this helps