[happyanimesh]

     Hi everybody..
     Well I am new to this forum, but I do have one question...
     What is the difference between molecular mass and formula unit mass.
     My text says:

     The average mass of one molecule of a compound in atomic mass unit is called
     molecular mass.
     The sum of the atomic masses of all the atoms in a formula unit of a compound is
     called the formula unit mass of the compound.

     But I am confused because the molecular and formula unit mass is the same.
     So are both the same??

[enahs]

You can treat them as the same.

Ionic compounds such as NaCl do not exist as individual molecules. Therefor no real molecular mass can be given. We can only write the simplest formula that shows the relative number of each kind of atom in a formula unit of the compound.

So to differentiate substances like NaCl that does not contain individual molecules, some chemist refer to their formula unit instead of the molecular weight/mass.

That said, most people do not even know the distinction, and tend to think it is just a difference in preference; as a result it ends up being used improperly.

But for everything you will be doing at your level, they are the same.