How to find the Formula Unit.

Let's say some one asked,
 74.5 grams of H2O, please find the formula unit.

To find the formula unit first we sort of do the same thing like when we find the Molar Mass.
So Hydrogen has 2 atoms,  and Oxygen has 1 atom.
Multiply The hydrogen atom  with the atomic mass and  do the same thing for Oxygen.

You would have:
H: 2 x1.0079= 2.0158
O: 1 x16= 16

Now add them
You should have 18.0158.

Then get the 74.5 and divide it by  18.0158.
 The answer is 4.1325

Finally the last step is multiplying the answer by Avogadro's Number ( 6.02 x 10^23).

You should get

24.88 x 10^23 formula units H2O.

I think you can figure this out easily.

Hope this helps