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Homework Help: Unit cell Density Problem

  1. Sep 29, 2011 #1
    Knowing the atomic weights of Na+ and Cl- are 23 and 35.5, respectively, calculate the density (g/cm^3) of NaCl crystal. Hint: Avogadro’s number N = 6.02 x 10^23 is the number of atoms in one mole (weight of 1 mole = atomic weight in grams)

    unit cell is Face centered
    unit cell contains a total of 4 Na+ atoms and 4Cl- atoms
    unit cell is a cube with length 'a'.

    so far I have found the mass of the total unit cell to be

    m = 4( 23 + 35.5) = 234g/mol

    and now im trying to find volume per mol but I'm not sure if I am doing this right...

    I found volume by cubing length 'a' because its a cube. I was wondering if I could avagadros number by the volume and then multiply that with 234g/mol to find the density
  2. jcsd
  3. Sep 30, 2011 #2
    A mass in atomic mass units divided by Avagadros number results in mass in grams.

    Assuming you want the density in grams per cubic centimeter, you may divide the mass of a unit cell converted to grams by the volume of a unit cell to converted to cubic centimeters.
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