Question

You extract the iron from a tablet and dilute that to 350.0 ml in a volumetric flask. You transfer 0.500 ml of this solution to a 75.0 ml volumetric flask and dilute this to volume. When you measure the concentration of this solution, you find that it is 1.87 ppm Fe. How many milligrams of Fe were in the tablet?

Answer 1

Fe sample ==> 350 mL.
0.50 mL ==> 75 mL = 1.87 ppm

1.87 ppm = 1.87 mg/L
How much Fe is in the 75 mL? That's 1.87 mg/L x 0.075 mL = 0.140 mg.
Therefore, the 0.5 mL must have cntained 0.140 mg Fe. Where did that come from? That 0.5 mL came from the 350 mL so there must have been 0.140 x (350/0.4) = ? mg Fe in the original sample.

Answer 2

Where did the 0.4 come from in your final calculation? Also your 0.075 should be L, not mL...