Homework Help: Chemistry

Posted by Ken on Sunday, November 27, 2011 at 3:49pm.

A 25 mL sample of waste water was treated with acid and SCN- to form an intense red complex. This complex was diluted to 100.0 mL and put in a variable pathlength cuvet. For comparison, a 10.0 mL reference sample of 6.8 x 10-4 M Fe3+ was treated with acid and SCN-, and diluted to 50.0 mL. This diluted reference sample was placed in a 1.00 cm pathlength cuvet. The variable pathlength cuvet had to be adjusted to 2.48 cm for the test solution in it to match the absorbance of the diluted reference sample. What is the molarity of Fe3+ in the waste water?

I know I'm suppose to use Beer's law to help solve this, but I'm not sure exactly how to start this problem. Answer is 2.19 x 10^-4 M

Chemistry - DrBob222, Sunday, November 27, 2011 at 4:16pm
The reference(which I'm calling the standard) sample is 6.8E-4M and 10.00 mL is diluted to 50.0 mL. So the reference standard is 6.8E-4M x (10.0 mL/50.0 mL) = ?M
Lstd x M std = Lsample x Msample
1.00cm x ?M std = 2.48cm x M sample
Solve for M sample.

Then look to see how the sample was treated. You had a 25 mL sample to which the Fe^3+ was added, then diluted to 100; therefore, the M of the sample in the cuvette must be multiplied by (100/25). I agree with the answer.
Post your work if you get stuck.

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