You are to convert 3.55 moles of
NaCl to formula units. Always remember that the
Avogadro’s number is 6.023*10^23 units per mole of a substance. Units could be
atoms, molecules or formula units. So, just multiply 3.55 moles
to 6.023*10^23 formula units per mole and you will get
2.138x10^24 formula units of NaCl.
