What does molecular weight have to do with density.?

Question

I know the density of water is 1, and its mm = 18. Octane has a density of .703 or something, and its mm = 114. Can someone explain how to determine the density of each?

Answer 1

Density = mass / volume, or D=m/V

You need some way to connect the mass of one mole to the volume of one mole.  If the substance is a gas, then it is very easy.

1 mole = molar mass (AKA molecular weight)

1 mole of any gas at STP = 22.4L

Therefore, the density of any gas at STP is the molar mass over 22.4L

I.e.  The density of oxygen gas, O2, at STP is ...

D = m / V = (32 g / mol) / (22.4 L / mol) = 1.43 g / L

Answer 2

The units of density are g/mL and the units of molar mass are grams/mole. You can compute the density of something by using a scale, measuring how much fluid it displaces, or using the geometry of the substance and calculus, but you can't compute density from molar mass. By looking at the units you can clearly see that there is no relationship whatsoever. The densities of common substances have already been calculated and you can look them up just as you can look up the molar mass of every known substance on the periodic table.

Answer 3

density has nothing to do with molecular weight.  molecular weight is based on the intermolecular forces between the molecules.  You will find no calculation or concept based solely on MW to find what you are looking for.