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Homework Help: Bond energy between two water molecules

  1. Nov 12, 2013 #1
    1. The problem statement, all variables and given/known data

    Estimate the bond energy between two water molcules (liquid) in the following way

    A: From the density of water and its molecular weight calculate the average distance between the molecules

    B:hence show that there are 10^19 molcules per m^2 of surace

    C:from the suraface tension of water, assuming this to be the same as the suraface energy, fine the value of U_0

    2. Relevant equations

    [tex]N_s ≈ \frac{1}{r_0^2} \\
    \frac{1}{4}nU_0N_s \\
    \gamma = \frac{F}{L} \\

    3. The attempt at a solution

    I am stuck straight away on part A. I know the density of water (1000 kg/m^3) and the molecular weight of H2O is 18 but have no idea how these are supposed to help me calculate the average distance between the molecules.

    I am not looking for the answer but any hints on where to start would be very much appreciated.
  2. jcsd
  3. Nov 12, 2013 #2
    The volume of one molecule (in m^3):
    V_1 = \frac {μ}{ρ N_A}
    where ## μ = 18×10^3 kg/mole##
    ## ρ = 10^3 kg/m^3##
    ## N_A = 6×10^{23} mole^{-1}##
  4. Nov 12, 2013 #3
    OK Thank you, what is mu called? (For example rho is density)
  5. Nov 12, 2013 #4


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    I think it's supposed to be the molar mass of water (2xH+1xO = 18). But I believe it should be 18g/mol, the molar unit having been defined in terms of grammes before the old cgs units were replaced by MKS units. So that's 18 10-3 kg/mol, not 18 10+3 kg/mol.
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