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Homework Help: Density of Gold

  1. Oct 17, 2006 #1
    I thought I understood this problem, but my answer is wrong.

    "In a sample of 18-karat gold, 75 percent of the total mass is pure gold, while the rest is typically 16 percent silver and 9 percent copper. If the density of pure gold is 19.3 g/cm^3, while the densities of silver and copper are respectively 10.5 g/cm^3 and 8.90 g/cm^3, what is the overall density of this alloy of 18-karat gold?"

    So I understand that 18-karat gold is 75% gold because it is 18 parts out of 24, with the other 6 parts made up of silver and copper, equaling 100%. So it made sense to me, to do this:
    Gold: (0.75)(19.3 g/cm^3) = 14.475 g/cm^3
    Silver: (0.16)(10.5 g/cm^3) = 1.68 g/cm^3
    Copper: (0.09)(8.9 g/cm^3) = 0.801 g/cm^3
    Then I added them: 16.96 = 17.0 g/cm^3

    This is wrong, isn't that what it is asking for? Did I understand the question wrong?
     
  2. jcsd
  3. Oct 17, 2006 #2

    Fermat

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    75% of the mass is gold, but you have been working with 75% of volume.
     
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