1. Iron containing tablet

The amount of iron in the tablet is determined by using the Beer's Law plot determined in Part 1 of the exercise.  The Beer's law plot is used to determine the concentration of a sample made by diluting a solution containing the entire contents of the tablet.

The computations relate only to conversion of the measured concentration of iron in the diluted solution back to the amount of iron in the tablet.

Note that, since we are interested in its total iron content, we do not need the weight of the tablet in this exercise.

Denote the total amount of iron in the tablet in milligrams by Z.

1. The tablet is dissolved in hydrochloric acid and ultimately the volume of the solution obtained is adjusted to 100.0 mL in a volumetric flask. (Solution A)

Solution A consists of Z mg of Fe in 100.0 mL of solution (0.1000 L). 
[ Concentration of A =  Z  mg Fe / 100.0 mL ]

2. A 5.00 mL aliquot of Solution A is diluted to 100.0 mL to make Solution B.  

Solution B contains a total ( 5.00 / 100.0 ) Z  =  Z / 20.0 mg of iron in 100.0 mL of solution. 
[ Concentration of B = ( Z / 20.0 ) mg Fe / 100 mL ]

3. A 10.00 mL aliquot of Solution B is diluted to 100.0 mL (by adding the required reagent solutions and water) to make Solution C. 

Solution C contains a total of ( 10.00 / 100.0 ) Z = Z / 10.00 of the iron in Solution B.
[ Concentration of C = (Z / 200.) mg Fe / 100 mL ]

Example:

Suppose the concentration of iron in Solution C is determined, from the Beer's Law Plot, to be 1.87 mg of Fe / L.  How much iron was in the original tablet?

Solution C contains 1 / 200. of the amount in the tablet.  Therefore, the weight of iron in the original tablet was 0.187 X 200. = 37.4 mg

Unlike the tablets, the iron containing unknowns are solutions and you are asked to report their concentrations, not the total amount of iron in the sample.  The solutions are, however, too concentrated to measure their absorbances directly.  They must, therefore, be diluted.  They are, in fact, diluted by taking a 10.00 mL aliquot of the unknown and diluting it to 100.0 mL in a volumetric flask -- a ten-fold dilution.

The concentration of the resulting solution is 10.00 / 100.0 = 1 / 10.00 the concentration of the unknown.

If the concentration of the diluted unknown is found, from the Beer's Law plot, to be 3.6 mg Fe / L, the unknown must have had a concentration exactly 10.00 times as large, or 36 mg Fe / L.