# Molecule size of water and oxygen

1. ### Coda

8
Which size of the molecule is bigger? Water or oxygen?

Can someone state out the size unit of these molecules

And, can I find a meterial to seperate this 2 matter but let one of it pass through the other? (Osmosis effect)

Really appreaciate who can help out here, thx

2. ### wywong

114
The density of water is 1kg/L and that of liquid oxygen is 1.14kg/L. The molecular weights are 18 and 32 respectively. So I reckon the volume of an oxygen molecule is about 32/(18*1.14)=1.56 times that of water. The exact volume of each molecule can be calculated using Avagadro's constant and the above data. If the molecules are assumed spherical, apply a factor of $$\frac{\pi}{3\sqrt 2}$$ for closely packed spheres and you can find out the radius.

Can't help with materials for osmosis.

Wai Wong

3. ### wywong

114
I think to find the material for your osmosis purpose, calculating the 'sizes' of the molecules is not the right approach because they are of comparable volume but different geometrical shapes - oxygen molecule like a dumbbell and water molecule like a V shape. If a material have holes slightly bigger than an oxygen atom it should theoretically allow the passage of oxygen molecules but not water, but actually very few oxygen molecules are aligned in the right direction to get through. Also atom and molecules have fuzzy boundaries and the electron cloud of oxygen/water molecules will interact with that of the material around the hole, making passage difficult. If some or all holes are any larger, both molecules can get through. So a traditional approach of finding the membrane material may not work.

I suspect permeable membrane with certain chemical properties (e.g. hydrophobic) may do the trick. You may want to seek help in chemistry forums.

Wai Wong

8