What is the difference in calculating formula and molecular mass?

Question

In my book it says the same for both. is there even a difference?

Answer 1

You calculate both the same way, but technically a formula mass refers to an ionic substance which can form large crystals (NaCl). We calculate the mass of one formula unit from the crystal so we don't have to add all the units in the crystal. Molecular mass refers to molecular substances which exist as separate molecules (CO2).

Answer 2

formula mass -ionic compounds
Molecular mass-covelent componds