Another chem problem

1. Sep 29, 2007

higherme

here is the question:

The entire contents of one of the Erlenmeyer flasks used in the oxalate-permanganate titration in Experiment 8 was poured into a 250 mL volumetric flask. The volume was made up to 250 mL. A 20 mL portion of the solution was placed in a 100 mL volumetric flask and 20 mL 0.01 M thiocyanate, and 60 mL 0.5 M nitric acid was added. The absorbance of the solution was measured and the intersection plotted on the calibration graph. The concentration of Fe(III) in the solution was found to be 2.410E-4 M. Calculate the mass of iron in the Erlenmeyer flask from Experiment 8. Enter your answer in scientific notation with three significant figures.

My attempt:

calculate the amount of moles in that 100mL flask. so,
since we know the concentration of FeIII is 2.410E-4 M
I multiplied that by the volume of the flask which is 100 mL = 0.1 L
I got 2.41E-5 moles of FeIII

Then i took that mole of FeIII and multiplied it by molar mass of FeIII which is 55.845 g/mol ( unless there is another mass cuz i know there are Fe II and Fe III)

now i got 1.35 E-3 g of FeIII which is in the the original erlenmeyer flask??

well, my answer was wrong according to the computer.

2. Sep 29, 2007

higherme

actually, i know what i calculated up there....
the is the grams of FeIII in the 100 ml flask, what i need is the g of FeIII in the 250mL flask.
but...how would i find that from what i have?
M1V1 = M2V2 ??... what about that 20mL that was transfered ?? so confused

3. Sep 30, 2007

higherme

4. Sep 30, 2007

symbolipoint

So far, you just found that 1.35e-3 grams of Fe(III) is in THE 100 ML VOLUMETRIC FLASK - NOT THE ERLENMEYER FLASKS.

Track that to the 20 ml. used for preparing the volumetric solution. This 20 ml came from the 250 ml. solution in the second erlenmeyer flask.
$$$1.35 \times 10^{ - 3} \,grams\,portionTaken\,\, \times \,\frac{{250\,ml\,originalSample}}{{20\,ml\,portionTaken}}\, = \,grams\,originalSample,\,Fe(III)$$$
The transfer of the original solution in the first erlenmeyer to the 250ml. preparation in the second erlenmeyer flask should not distract you from seeing the dilution progression. The lower accuracy in that step is probably minor. Not too serious a problem for your instructional purposes.

5. Sep 30, 2007

higherme

i still don't really get why (250 mL original sample/ 20 mL portion taken) part....

6. Sep 30, 2007

symbolipoint

Twenty milliliters of solution was taken from the 250 ml solution in the erlenmeyer flask. The amount of iron was 250/20 times greater than the amount of iron in the 20 ml which was taken.