# Calculating Molar Volume from crystal volume

I am trying to calculate molar volume of certain crystal lattices to be used for crystal growth kinetics calculations. The information I have is the crystal volume & crystal lattices. For example the crystal $\ce{Cu41Sn11}$ has the structure cF416 and the information taken from a certain crystal database is as follows:

Atom Formula: Cu41.5Sn10.5

Cell volume: 5.81258 nm3

Cell density (calculated): 8.85 Mg m-3

Now I have tried 2 approaches to it.

1. First was the wikipedia formula for molar volumes

Vm= NAVcell / Z

molar volume = (Avorgados number x crystal cell volume) / number of formula units in a crystal

This gives the following output for the above mentioned crystal

Molar Volume = 6.022 x 1023 x 5.81258 x 10 -27 / 51

Molar volume = 6.731 x 10 -5

1. The second one is i figured out the formula

Molar volume = Molar Mass of crystal / Cell Density

So in this case:

Molar mass of crystal $\ce{Cu41Sn11}$ = 41 x 63.546 + 11 x 118.71 = 3911.2 gms (3.9112 Kg)

Molar Volume = 3.9112 / 8850 = 4.41944 x 10 -4

These two approaches yield two different results, so i wanted to make sure where am i making the mistake?

In case you're interested in the crystal database, here it is: 