# Bond energy between two water molecules

1. Nov 12, 2013

### FaraDazed

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

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

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. Nov 12, 2013

### GregoryS

The volume of one molecule (in m^3):
$$V_1 = \frac {μ}{ρ N_A}$$
where $μ = 18×10^3 kg/mole$
$ρ = 10^3 kg/m^3$
$N_A = 6×10^{23} mole^{-1}$

3. Nov 12, 2013

### FaraDazed

OK Thank you, what is mu called? (For example rho is density)

4. Nov 12, 2013

### haruspex

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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