A salt crystal has a mass of 0.20 mg how many nacl formula units does it contain

This is a unit conversion problem. NaCl formula units are the same as the number of molecules that are pair bonded together. 0.20 milligrams x (1 gram / 1000 mg) x (1 mole/58.44277 grams of NaCl) x (6.022140857e 23 formula units (Avogadro's number) / 1 mole) = 2.06087Ă-10^18 formula units As they go, the units cancel each other out when the problem is constructed this way, leaving you with formula units.