What is the difference between molecular weight and formula weight?

Question

What is the difference between molecular weight and formula weight?

Answer 1

molecular weight is the "mass" (not weight) of one molecule of a substance.  Mass and weight are often interchangeable for this topic.

Formula weight is the "mass" of one unit of an ionic compound.  ionic compounds are not molecules so technically they do not have a "molecular" weight.  

examples:  

H2O is a molecule and has a molecular weight of 18 g/mole

NaCl is not a molecule.  It has no "molecular weight".  It exists in a lattice of Na and Cl atoms.  The formula weight for NaCl is 58.44 g/mole and is the mass of 1 Na + 1 Cl since the ratio of Na to Cl in the lattice is 1:1.  

Na2SO4 is another ionic compound.   Again, it isn't a molecule and therefore has no molecular weight.   It exists as a lattice of Na and SO4 atoms of ratio 2 : 1 and therefore has a formula weight of 2 x 23 + 1 x 96 = 142 g/mole

***** Norrie *****

Where exactly did you get the idea that ionic compounds don't contain elements?  Did I say that?  Nope.  I wouldn't.  Ionic compounds most certainly do contain elements.  They are NOT, however, MOLECULES and therefore do not have a MOLECULAR mass.   Which is the whole point behind the distinction between molecular and formula weights and is the answer to this question.  Molecular weight applies to "molecules" while formula weight applies to "ionic compounds".  They are not the same thing.

As to a vendetta  ?????.   Are you nuts?  You answered a question incorrectly.  Received what, 3 thumbs down for your incorrect answer and are trying to defend rather than correct yourself.   In the mean time you start calling others names and you and I both know what you meant by AH.   Now how childish is that?  An old man calling a high school student an AH or a Pratt or a whatever else you can come up with just because someone gives a wrong answer you wrote on a yahoo board a thumbs down?  Would you still feel it was OK if it was one of your grandchildren being called names?  Isn't it about time you grew up?

So here's a suggestion.   Do a little research before writing answers to questions that you aren't sure about.  Then maybe you'll be correct more often and receive less thumbs downs.   And of course, the name calling ALWAYS guarantees a thumbs down from me.  So here's your thumbs down from me to you for calling another yahoo member an AH.  Now you have 4.

Answer 2

it's the same..molecular weight for molecular compound, formula weight for ionic compund..still, the calculation's the same

Answer 3

It's simply 2 ways of expressing it.

Molecular Formula for water = H2O with a Molecular, Formula weight (now expressed as 'Mass'), of 18g/mole.

Isn't this answer exactly the same as the other ???

Why a thumbs down ?? AH

So, ionic compounds don't consist of Elements eh? and, therefore the elements don't possess an atomic mass ??

AH means Aaaa..Haaa..why ? and, why don't you give it up m w...what have I done to you ?

This vendetta of yours is totally unwarranted, unfair and unnecessary.