Beer's law and absorbance using a spectrophotometer?

Solution X is made by dissolving 190 mg of synthesized apsirin in 250.0 mL of water. Solution Y is made by diluting 5.00 ML of solution X to 100.0 mL with .02 M FeCl3-KCl-HCl solution. The Beer's law constant for the Fe-salicylate complex is 1350L/mol. Solution X shows an absorbance of .135/. What is the percent of ASA in the synthesized aspirin? Thanks in advance for anyone who has an idea!

Answer:
The concentration of the FeCl3-KCl-HCl solution is not significant in the calculation. You are only interested in the dilution from Solution X to Solution Y.

Solution X = 190 mg/250 ml = 0.190 g/0.250L = 0.76 g/L

Aspirin MW = 180.160 g/mole

0.76 g/L * mole/180.16 g = 4.21*10-3 M

you took 5 ml of this and diluted it to 100ml

5ml/100ml * 4.21 *10-3 M

= 2.11 * 10-4 M = final concentration of aspirin Fe complex

NOW the beer law stuff - it's as easy as ABC

A = abc, A= absorbance, a = absorbance coefficient, b = cell length, c = concentration

From the data A=0.135 and a = 1350 L/mole, there is no mention of b - so I will have to assume it is unity. This gives

c = 0.135/(1350L/mole) = 1 * 10-4 M

From your original calculation to concentration should have been 2.11* 10-4 M

so the % complex in the solution is 1 * 10-4 M/ 2.11*10-4 M *100 = 47.39%

First of all Susan, stay away from the beer. It is too early to be drinking. The beer law states that the more beer you drink, the drunker you will become.

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Beer's law and absorbance using a spectrophotometer?

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