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.
