How many formula units are in 3.6 grams of nacl?

Use stoichiometry.

3.5g-> moles

22.99+16+1.0079=39.99

Now 3.5/39.99 = .088 moles of NaOH

Now you can use avagadros number 6.022x10^23

.088 moles -> formula units

.088/6.022x10^23

You should end up with

1.46x10-25 formula units.

Yeah, I know, Chem sucks huh?