Relationship between number of things, N, and number of moles, n.

1)  N  = NA n     where: NA = 6.022 x 1023 mol-1


Relationship between moles and mass
2)
where
M   is the molar mass (from the periodic chart) with units of g mol-1
m   is the mass in g
 n   is the number of moles  

The molar mass is read off the periodic chart.  If the species is a molecule (or formula unit), the molar mass is the sum of the molar masses of the atoms in the molecule (or formula unit).

Given mass how would one solve for number atoms or molecules?  It should be obvious that one could only use the second equation (no choice) first followed by first equation to give numbers.