Hi was looking to get some help with an experiment. Currently I trying to determine the amount iron in

Question

cereal.

And was hoping to get some help with one of the questions I have been given.

First off I am the recorded absorbance across the region 470 – 490 nm for the 3.00 mg L–1 iron(III) standard solution constructed in Part 1 above yielded a λmax of 478 nm with an absorbance of 0.674 AU on an instrument with a 1 cm path length.

I have calculated the molar absorptivity to be e= 0.225 L mg^-1 cm^-1 from e = A/bc (a variant of Beer's Law).

I then have been given a set of values for %transmittance of 80, 60, 50, 40, 30, and 20. I have calculated their A values using A = -log (%T/100).

However they want me to find:
(1) the amount of [Fe3+] required (mg L–1);
(2) the quantity Fe3+ required in 50 mL (mg); and
(3) Volume Fe3+ standard solution required (mL)

For (1) I am guessing that I need to use A = ebc and solve for c? (2) I am lost on and same with (3).

Any help you could give would be greatly appreciated.

Answer 1

I.e. Say for 20% T:

A = - log (%T/100) = - log (20/100)
=0.69897 = 0.699

Then using c = A/bc:

c = 0.699/(0.225 x 1)
= 3.11

Does that mean the amount for (2) is 3.11 mg L^-1?

Answer 2

And then how do I get (3)?

Is it a matter of going c = 3.11; where 0.15 mg in 50mL -> 0.15/50 = 0.003 = 3ppm. So 3.11mg/50mL = 0.0622 = 62.2 ppm?

Answer 3

Should add that this is the question as well, sorry:

"Calculate the absorbances and using this and the measured molar absorptivity, predict the
concentrations of iron(III) and therefore volumes of the 30 mg L–1 iron(III) standard solution
required to make your appropriate range of standards."

Answer 4

I am terribly confused but I can make the following comments.
The molar absorptivity is USUALLY given in mols/L and that's the way it is tabulated in reference tables; however, your 0.225 is ok as long as you show the units that go with it as you've done.

Then for the questions:
However they want me to find:
(1) the amount of [Fe3+] required (mg L–1); in what? an unknown? some solution which you haven't specified? My best guess is that those values you mention are concentration Fe for the percent T listed. I think you are to draw a working curve as Absorbance vs concn and read the concentration you wat from that. But that's just a guess since I don't now what is being measured.
(2) the quantity Fe3+ required in 50 mL (mg); and For this if you have Fe in mg/L you multiply by 0.050 L to find Fe in 50 mL
(3) Volume Fe3+ standard solution required (mL)I have no idea what this means. Required for what?

Sorry I'm not more help but the post is just not clear what you are doing and to what. If you can make a post with one question at a time it might make more sense.