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Homework Help: How to solve for viscosity using Stoke's law

  1. Feb 19, 2010 #1
    I'm having a bit of trouble with some simple viscosity lab problem. The results are not of my own, so this could be some procedural error. We've used glycerol as the liquid medium at 3 different temperatures (0C, 10C, and 20C) with three different metal spheres, one of them being steel. All we are required to do is calculate the viscosity and temperature dependence of the glycerol using this "falling-sphere" method. Stoke's law is given to us:
    η=[2g(R^2)(ρ_S-ρ_L)t]/9L

    There's also a correction for a cylindrical tube, where the medium is, inserted into the denominator
    η=[2g(R^2)(ρ_S-ρ_L)t]/[9L(1+2.4x)(1+1.65y)]
    Where x is the ratio of the sphere's diameter to the walls of the cylinder (graduated cylinder) and y is the ratio of the sphere's diameter to the total height of the liquid in the graduated cylinder.
    These are what I have for results and constants:
    Steel ρ = 7860 kg/m^3
    Glycerol ρ = 1259 kg/m^3
    t = 2.3 s
    L = 0.05m
    R = 8.314 J/molK
    g = 9.81 m/s^2

    This trial was run with the glycerol medium at 0C and the given density was at 25C, so I corrected the density of glycerol using the coefficient of expansion (0.0005).
    The accepted value for the viscosity of glycerol is 1.5 Pa·s.
    I ended up getting 1.46E7 [(J/molK)^2 kg/m^3]
    And yes, I'm terrible at converting these units.
    I can't see where I am going wrong with this. Any help/hints would be greatly appreciated so I can move on with the other problems.
     
  2. jcsd
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