Hi was looking to get some help with an experiment. Currently I trying to determine the amount iron in
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.
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?
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?
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."
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.
Sure, I can help you with that.
To find the amount of [Fe3+] required (mg L–1), you can rearrange the Beer's Law equation, A = ebc, to solve for c. Since you have already calculated the molar absorptivity (e) and the absorbance (A) values for each %transmittance, you can use the equation in this form:
c = A / (e * b)
Where:
c = concentration of Fe3+ (mg L–1)
A = absorbance (calculated from %transmittance)
e = molar absorptivity (0.225 L mg^-1 cm^-1)
b = path length of the cuvette (1 cm)
Now, let's move on to the next two parts of the question.
To find the quantity of Fe3+ required in 50 mL (mg), you need to calculate the mass of Fe3+ present in the 50 mL solution. This can be done using the following formula:
Mass (mg) = concentration (mg L-1) * volume (L)
In this case, the volume is 50 mL, which needs to be converted to liters (L) by dividing it by 1000. Then, you can use the concentration you calculated in part (1) as the concentration value.
For the third part, to find the volume of the Fe3+ standard solution required (mL), you can rearrange the formula from part (2) to solve for volume:
Volume (mL) = Quantity (mg) / Concentration (mg L-1)
Again, use the concentration calculated in part (1) as the concentration value and the mass of Fe3+ required as the quantity value.
By following these steps, you should be able to find the answers to parts (1), (2), and (3) of your question. Let me know if you need any further clarification.