What is the difference between molecular mass and formula unit mass???

The formula mass (formula weight) of a molecule is the sum of the atomicweights of the atoms in its empiricalformula. The molecular mass(molecular weight) of a molecule is its average mass as calculated by adding together the atomic weights of the atoms in the molecular formula

Formula mass is defined as the sum of atomic masses of the ions present in the formula unit of an ionic compound while molecular mass is the sum of the atomic masses of the atoms in a molecule. Formula mass is more general term than molecular mass and maybe used in referring to all compounds but not the molecular mass.

In more detail.
There is a cyclopean difference between molecular mass and formula mass.

Molecular mass is the mass of a given molecule or compound but now the clinch here is that it remains neutral despite the presence of zillions of molecules. For ex: Molecular mass of CO2 = 12*1 + 16*2=44.
2.But, now the formula mass is something different. IT IS THE MOLECULAR MASS OF THE WHOLE COMPOUND CONTAINING X NO. OF MOLECULES OF THAT COMPOUND. Let me clarify it with example: Like we have to find the formula mass of 10Na2SO4.H2O= 10[23*2+32+16*4+2*1+16*1] = 10*180= 1800.

So, it is pretty clear that formula mass is ‘x’ times the molecular mass of that compound.