1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Modify the Radius

  1. Nov 23, 2007 #1
    1. The problem statement, all variables and given/known data

    The atomic mass of iron (Fe) is approximately 56 grams per mole.

    (a) Assume each iron atom in a bar of iron takes up a volume equal to a sphere of radius 2.35×10-10 m. Calculate the density of iron based on this model.

    (b) The actual density of iron is 7870 kg/m^3. Modify the atomic radius in (a) such that the calculated density agrees with the actual value.

    2. Relevant equations

    Density = mass/(4/3)[tex]\pi[/tex]r^3 ( D=m/V )

    3. The attempt at a solution

    I figured out a) to be 1.71E3 kg/m^3. However, I can't figure out b).

    The new radius is 1.41E-9 from what I calculated , but it is not the right answer.
    Last edited: Nov 23, 2007
  2. jcsd
  3. Nov 24, 2007 #2
    Does anyone know??? Can anyone help???
  4. Nov 24, 2007 #3
    You just need to find how many atoms are in one mole.

    One mole weighs 56 grams,and density is

    [tex]\rho=\frac{56 g}{Nv_0}[/tex], where [tex]v_o[/tex] is a volume of one atom.
  5. Nov 24, 2007 #4
    Thanks azat for making me realize my mistake. I was calculating the actual mass to be in g and not kg (I had already converted N) while leaving the density in kg/m^3. Thank you, it turns out to be 1.41E-10 m.
    Last edited: Nov 24, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook