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Homework Help: Chemical composition of the earth - Astronomy/Chem question

  1. Jan 28, 2009 #1
    1. The problem statement, all variables and given/known data

    Calculate and tabulate the fractions of the total mass of the Earth which would be
    accounted for by O, Na, Mg, Al, Si, S, Ca and Fe under the following hypothese.
    i) All the elements above are fully oxidized, except that no S is accreted and the
    Fe is entirely metallic (that is, brings no O with it)

    you may neglect H, He, C, N, Ne, Ar and unbound oxygen (which do not
    condense at the temperatures in question) and all elements less abundant than sodium. In
    your answer, please tabulate the mass fractions of each elements (including O), not the
    mass fractions of the oxides. You may assume the relevant oxides are Na2O, MgO, Al2O3,
    SiO2, and CaO.

    2. Relevant equations

    There is a table that has abundances of elemental atoms (per 1000 Si atoms)
    Na 60
    O 19000
    Fe 890
    Mg 1070
    Al 83
    Si 1000 (obviously)
    Ca 65

    the table as relevent molar masses as well

    3. The attempt at a solution

    What I did was assume that:

    mass of (Na2O + MgO + Al2O3 + SiO2 + CaO + Fe) = Mearth
    since all the abundances are given per Si atoms, I want to put everything in terms of per Si atom. I started with the lone Fe, and calculated that 1 Si atom -> 0.89 Fe atoms -> 1.48e-24mol -> 8.26e-23 g/Si atom. Good, done

    but now is where the chemistry comes in -> how do I do this same process for the oxides like CaO and Al2O3. Someone please help
  2. jcsd
  3. Jan 28, 2009 #2
    I'm trying to follow what the question is asking; it doesn't seem perfectly clear. I think you should take all the oxides and count those towards your abundances. So, for example you have 19000 Oxygen atoms per Silicon atom. And oxygen appears in CaO, Al2O3, Na2O, MgO,Al2O3, and SiO2. So maybe you need to divide the available abundance of oxygen up into these oxides.

    But this question is somewhat outside of my experience.
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