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Homework Help: Average diameters of colloids

  1. Jan 22, 2009 #1
    A dispersion of spherical polymer particles contains Np = 4•1013 particles/dm3. The total surface of the particles in 1 dm3 is Asp = 820 m2.

    a) What kind of average diameter can we calculate. Calculate this average diameter.

    b) If we know that the dispersion contains 60 vol% water, we can calculate another type of average diameter. Which? Find it!

    a) Surface average.

    [tex]\bar{d_s} = \sqrt{\frac{\Sigma n_i d_i^2}{\Sigma n_i}}[/tex]

    b) Volume average.

    [tex]\bar{d_s} = \sqrt{\frac{\Sigma n_i d_i^3}{\Sigma n_i}}[/tex]

    I don't know how to incorporate the data into the formulas.
  2. jcsd
  3. Jan 23, 2009 #2
    Can't you just work it out by taking the surface area of a sphere and working back to get the radius?

    You know the total surface area of the total particles in 1 litre, and you know how many particles there are in 1 litre, so from that you can work out the surface area of 1 particle.
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