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Need Help in Calculating MW of Unknown

  1. Mar 23, 2004 #1
    This is my first time posting here, I hope I get some help:-)
    Well here is my Information i have:

    Weight of Unknown= 0.2g
    # of mols of Fe(II)= 0.0005 mol

    Question: Assuming that the number of atoms in Fe in the unknown compound is one and the MW(Fe)= 55.85 g/mol calculate the molecular weight og the unknown iron compound.

    I've tried many things but so far i'm stuck
    Any help would be great!

    I also have to calculate the Mass of Fe(II) I was wondering if this is the correct method i'm using:
    0.0005 mol of Fe(II) x 55.85 g/mol (Fe)= 0.027925 g?

    Also need to find the % Fe in the Unknown, now would the calculation be:
    (0.027925 g /0.2 g) x 100?

    Thanx again
    Last edited: Mar 23, 2004
  2. jcsd
  3. Mar 23, 2004 #2
    Well everything you have said looks good. Now you just have to think if you have a total % of the Fe in the mass. Then you know what percentage of the MW is due to the Fe. Thus you can then accomodate to figure out the other element(s) percentage of the MW and figure it out.

    hope this helps

  4. Mar 24, 2004 #3


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    Okay, since they tell you how many moles of iron are in 0.2 g of the compound, and they tell you that each molecule of that compound has only one iron atom in it, you also know how many moles of the molecule you have (0.0005). The molecular weight of iron is irrelevant to the first part of the question (you needed it for the later parts of the questions).

    So far then, you know that for this compound, there are 0.2g per 0.0005 mol. All you need to do now is multiply numerator and denominator by a number that will set the denominator to 1 and you'll have the # of g/mol. I'll leave you to do that for yourself.
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