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Homework Help: Density Of Metal In Alloy

  1. Sep 19, 2009 #1
    1. The problem statement, all variables and given/known data
    A Mg-Al alloy has a mass of 0.263 g and a density of 2.40 g/cm3. The density of pure Mg =
    1.74 g/cm3 and for pure Al = 2.70 g/cm3. Calculate the % Mg in the alloy.

    2. Relevant equations

    3. The attempt at a solution
    Well I'm not really sure where to do. I solved for the volume of the alloy, then tried to use it to determine the mass of the Mg. Once there I divided by the total mass and multiplied by 100 to get the wrong answer.

    How can I go about getting the mass of Mg in the alloy?
  2. jcsd
  3. Sep 20, 2009 #2


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    Staff: Mentor

    Assume that volume of the alloy is the sum of volumes and mass is the sum of masses. That gives two equations in two unknowns.

    In reality only one of these equations is correct - that is, mass is sum of masses. Volumes are not additive. But that's not your fault.

  4. Sep 20, 2009 #3
    Could you please elaborate on why volumes aren't additive in this case?4

    Also I'm getting nowhere fast.

    I wound up with 0.263g = MassMg + MassAl

    Then solved for MassAl like so:

    MassAl = 0.263 - MassMg

    Then tried to substitute it back in:

    0.263 = MassMg + 0.263 - MassMg

    Which just cancels itself out.
    Last edited: Sep 20, 2009
  5. Sep 20, 2009 #4


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    Staff: Mentor

    Where is the equation for sum of volumes? Note, that volumes can be expressed using mass and density.

    Volumes are not additive in general. If you mix 50 mL of water and 50 mL of ethanol you will get 96.4 mL of mixture.

  6. Sep 20, 2009 #5
    Ok. I finally got word back from my teacher, and I think I see what to do now. I'll post my resulting solution when I figure it out.
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