Whats the correct way to approach this concentration question?

Question

Part I. Preparation of a Fe(III)-oxalate complex: (4 pts)

a) Recorded mass of Fe(NH4)2(SO4)2●6H2O used: 1.3807 (g)

b) pH paper color upon addition of oxalic acid: Magenta

c) Does this color indicate an acidic, neutral, or basic solution: Acidic

 

 

g) Part II: Preparation of a Fe(II)-bpy standard: (4 pts)

a) Recorded mass of Fe(NH4)2(SO4)2●6H2O used: 0.1201 (g)

b) Calculate the [Fe] in the Fe(II)-bpy standard solution. Show your work and the final value in the space provided below.

The solution was initially diluted in a 100mL volumetric flask. Then 2mL were transfered into a 50mL volumetric flask whihc was then diluted to the calibration line (50mL final solution)

 

 

[Fe] = 0.000123 (M) This was what I got after my calculations but it's not working with the rest of my calculations I have to do after. Did I do something wrong?

Answer 1

Using a molar mass of 284.05 g/mole for Fe(NH4)2(SO4)2●6H2O

moles of Fe(NH4)2(SO4)2●6H2O = 0.1201 g x 1 mole/284.05 = 4.23x010^-4 moles

moles of Fe = 4.23x01^-4 moles since there is 1 mole of Fe per mole of Fe(NH4)2(SO4)2●6H2O

If I understand correctly, this was in 100 ml and then 2 ml diluted to 50 ml.  If that is the case, then ...

4.23x10^-4 mole/100 ml x 2 ml = 8.46x10^-6 moles

8.46x10^-6 moles/0.05 L = 1.69x10^-4 mole/L = 1.69x10^-4 M = 0.000169 M