How do I estimate the size of a water molecule containing avogadro's number of molecules?

Question

My specific question is...

The molecular weight of water is 18 amu. Therefore 18grams of water (=18mls H20) contains Avogadro's number of water molecules. What is the size (or dimensions) of a water molecule. I also need to clearly state my assumptions?!

How do i go about this problem?

Thanks for your help!

Jody

Answer 1

use the fact that the density of water is 1 gram / 1 cm^3

solve for the weight of 1 mol of water.

There are Avagadro's number of atoms per mol of water

18 grams of water per mol of water.

Use unit cancelation so that your final answer comes out in cm^3

Answer 2

Just divide 18 cm^3 by Avogadro's number. This gives you an "effective volume", where you assume the molecules are little cubes and are stacked one on another. You can look at these as being the limits of activity of a molecule before it interacts with another.

Answer 3

Its molecular diameter is about 2.75 Å. The atomic diameter can be determined from interpolation of the effective ionic radii of the isoelectronic ions (from crystal data) of O2- (2.80 Å), OH- (2.74 Å) and H3O+ (2.76 Å) [1167]. Coincidentally, this diameter is similar to the length of a hydrogen bond. The water molecule is smaller than ammonia or methane, with only H2 and HF being smaller molecules.

Answer 4

The assumptions are that you should disregard Van der Waals forces, Hydrogen bonds, etc...

18 mL Water / 6.02 x 10^23 = volume of one molecule of water given above faulty assumptions.