[spaghetti]

Hi! I was wondering if anyone can help me.  This is kind of a really stupid question and I am sure it is very easy, but I keep getting 2 different answers (211.4 & 117.4) and am not sure if I am doing the calculation right.  Here is the question.

Determine the blood serum level of cholesterol by the single standard method.  The absorbance data obtained are:
     A Standard = 0.350 (250mg/100mL)
   A Sample= 0.390
   A correction = 0.02
Calculate the concentration of cholesterol in mg/100 mL.  What would be the % error if A correction were not used?

Here is the formula.  total chol. conc (mg/100mL) = [A510X- Ac]/A510s   times Cs

Cs = 200mg/100mL
A510 stands for absorbance at 510 nm



Thanks so much in advance.

[Yggdrasil]

By the Beer-Lambert Law:

A = e*l*c

Where e is the absorptivity constant of the molecule, l is the length of the cuvet, and c is the concentration of your solution.  In your experiment, since you are only using cholesterol and the length of the cuvet remains constant throughout your readings, you can consider e*l a constant.

For your standard solution, you know A (0.350 - 0.020 = 0.330) and c (250mg/100mL) which allows you to calculate e*l (e*l = A/c = 0.00132).

Using this value of e*l, you can calculate the concentration of cholesterol in your sample, since you now know A (0.390 - 0.020 = 0.370) and e*l (0.00132).  By rearranging the Beer-Lambert Law, you get:

c = A/(e*l) = 280mg/100mL

[spaghetti]

Thanks for your help Yggdrasil.