Finding the volume of one water molecule

[Niles]

Homework Statement

Hi guys

I know the molar mass (i.e. g/mole) of H and O, but from this, then how do I find the volume of 1 water molecule? I know the densities of H and O as well, from which I can find the volume of H and O. Do I just add these, one of V_oxygen and two of V_hydrogen (I know the correct value is 2.99E-29 kg^3)? I tried doing this, but it did not give me the correct value.


Niles.

 

[Borek]

You better start with water density (1g/mL) and calculate how many molecules in 1mL. Assuming final volume of a molecule is a sum of volumes of atoms is a sure way of getting it wrong.

How come final answer is in kg³? You sure you didn't mean m³?

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[Niles]

I mixed it up - the volume is of course in m^3. Regarding the density, then what I wanted was a general way of doing it, because I might not always know the density of the molecule as a whole, but only each atomic-part of the molecule.

So, how should I do it? I can find V_H and V_O, but adding them as 2V_H + V_O doesn't give me the correct result.

 

[Borek]

So, how should I do it? I can find V_H and V_O, but adding them as 2V_H + V_O doesn't give me the correct result.

That's because this is a faulty approach. When atoms bind their volumes partially overlap.

And I am not aware on any general method that will let you calculate volume of the molecule from the volumes of atoms involved. It just doesn't want to be that easy. You can try to calculate molecule shape using Schroedingers equation and quantum chemistry methods, but it is not an easy task.

 

[Niles]

Ok, then the "density-method" is the way to go. Thanks!Niles.