[bpeck8]

Problem:

1,10 phenanthroline forms a bright red complex with iron(II). the complex has an analytical wavelength of 510nm and has a high molar absorptivity.  hydroxylamine hydrochloride is added to keep the iron in a reduced +2 state, and the pH is controlled using a acetate buffer. a series of iron(II) phenanthroline solutions were made by adding various ammounts of 4.300X10^-4M Fe^2+ to 5 mL each of hydroxylamine hydrochloride, acetate buffer, and 1,10 phenanthroline. distilled water was added to make the total volume of each solution 50.00mL. the percent transmittance of each solution was read, using a blank containing everything but the iron(II) solution. Data for the determination are..

volume of stock Solution, mL                        %T
1.00                                                       80.35
2.00                                                       64.71
5.00                                                       33.65
7.00                                                       21.78
9.00                                                       14.09
10.00                                                     11.32

Question 1) calculate the molar concentration of [Fe(phenanthroline)3]2+ in each solution

I know that the formula for this is:           number of moles=(molarity, mol L)(initial volume, L)

                                                then...

                                                         molarity of final solution =     number of moles       
                                                                                             final volume of solution, L

What i am having trouble with is what do i use for the initial volume and final volume? would i take (4.3x10^-4mol L)(.05L) to get number of moles, then take number of moles divide by lets say we are doing the first volume of stock solution .001L? Is what i am doing wrong? do i have to find out any other calculations?

Thanks,
Brent

[Borek]

You start with some volume of stock solution - what is amount of moles in this stock solution? Does it change after the solution is diluted to 50 mL?

[bpeck8]

I think i got it now, probably not though, but here is a try.

So for example lets say we are doing the second volume of stock solution.

2.00mL = .002L, use this first because this is what we are starting with. Makes sense.

(4.300*10^-4M)(.002L)=8.6*10^-7mol

50.00mL = .05L, then use this, because this contains everything that has been added together/mixed, which would be the final volume.

8.6*10^-7mol = 1.72*10^5M, so would this be the molar concentration? does the answer seem to look right?
    .05L

[Borek]

Looks OK, although you probably meant 10^-5, not 10⁵.

[bpeck8]

Looks OK, although you probably meant 10^-5, not 10⁵.

hehe yeah, meant to type that in.

Alright i have a few more questions and these questions still apply to the stuff i typed up in my original post.

question A)

a solution containing an unknown amount of iron (II) was treated with hydroxylamine hydrochloride, acetate buffer, and 1,10 phenanthroline as described above.  the percent transmittance of the sample at wavelength=510nm was 52.4 when read against a blank containing all but the iron (II) solution.  determine the molar concentration of iron (II) in the unknown.

question B)

calculate the molar absorptivity for [Fe(phenanthroline)3]2+


For part B, i know the equation and how to set it up and what to look for: A(absorbance)=E  (not sure how to write it, but its the molar absroptivity)b(pathlenght)c(concentration), here it is again with out all the descriptions, A=Ebc, now when rearranged to solve for molar absorptivity, A/bc=E. Now i already made a plot graph for absorbance vs. concentration(M). For this problem, would i use the concentration from Part A after its been calculated or do i look to the graph to get that along with absorbance? If the graph, do i just pick any two points?

For part A, I am not sure what to really do, i know that i need to find the unknown amount of iron(II), but not sure what else, and how to set the problem up and what all is needed.

Thanks,
Brent

[bpeck8]

never mind, figured it out, very simple and little calculations involved