Question

I am really having a hard time doing this question.

We were asked to determine the iron content of spinach in a solution.
We burned a 5.50g of spinach and added 10 ml of 2.0 M HCl in the spinach ashes. We mixed that and filtered the solution to removed the ashes. We used 5 ml of that solution and put it in a testube and added 5 ml of 1.5 M KSCN in the testube. We then use a spectrometer to determine its wavelenght, which is 0.178. I calculated the concentration and got 0.000023 M. We were asked to determine the mass of the iron in the spinach that we used , and determine the mg of iron per 100 g of spinach.

Answer 1

First I doubt you determined the wavelength. Probably the wavelength was set or you set it to some desired number. I suspect the 0.178 was the absorbance you read from the spectrometer. Any, one of that really matters.
So M = 2.3E-5 what? Is that 2.3E-5 M Fe. I will assume so. That is 2.3E-5 mols/L and that is 2.3E-5 mols x 55.85 g Fe/mol = approx 1.3E-3 g Fe/L or 1.3 mg Fe/L and that is in the final 5 mL sample you took to make the measurement. So mg Fe in the 5 mL measured sample is 1.3 mg Fe/L x 0.005 L = 6.5E-3 mg Fe. What mass spinach was in the 5 mL? That's 5.5 g/2 or 2.75 g. Now you have mg Fe in 2.75 g sample. Convert that to mg Fe in 100 g spinach.

Answer 2

Im not sure how to convert the mg Fe in 100 g of spinach.

Do i just multiply the mg Fe to 100 g or I have to divide the mg Fe by the spinach mass (which is 5.50 g before i burned it) and then multiply it by 100?