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Homework Help: 8 million uranium atoms in milligrams?

  1. Aug 17, 2006 #1
    Where i live in the UK all the A-level students have just had their results. It has been said on the news that a new A* level will be added to the A levels to differentiate between the bright and the very bright and that an example of an A* question would be: 8 million uranium atoms in milligrams.
    Just wondering how you would go about working this out. The way i thought it would be done is by working out the weight of an individual atom by proton, neutron number etc and multiplying by 8 million. The only problem is i have no idea how much a proton or neutron weighs or how to convert the impossibly small mass to a metric measurement.
    I would be very grateful to anyone who can shed any light on the matter

  2. jcsd
  3. Aug 17, 2006 #2
    From http://www.bartleby.com/65/at/atomMasU.html

    An Atomic Mass Unit [tex]\approx 1.66*10^{-24}[/tex]grams
    [tex]\approx 1.66*10^{-21}[/tex]milligrams

    I see Uranium (U) has an atomic mass of 238.07, so...

    [tex]8*10^6 * 238.07 * 1.66*10^{-21} = 3.1615*10^{-12}[/tex]

    Therefore 8 million uranium atoms weighs approximately [tex]3.1615*10^{-12}[/tex] milligrams.
  4. Aug 17, 2006 #3
    Thanks for the relpy. I understand it now

  5. Aug 18, 2006 #4
    Another way would be the use of Avogadro's number.
  6. Aug 20, 2006 #5
    I think you're overcomplicating this.

    238.07g = 1 Mol of Uranium, so 237.07/(6*10^23) = 1 Uranium Atom

    So (237.07*(8*10^6))/(6*10^23) = the amount he wants. Which is 3.16093333*10^-15 grams or 3.16093333*10^-12 milligrams (give or take a bit, because it's actually 6.022*10^23).

    It's much more simple, and you can figure it out with elementary chemistry knowledge.
    Last edited: Aug 20, 2006
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