How to you convert Molecular Mass (g/mole) to the total number of molecules/atoms?

First of all, you have to know the mass of the substance in grams.
Number of moles = (Mass in g) / (Molecular mass in g/mol)

Avogadro constant = 6.022 × 10²³
Total number of molecules = (Number of moles) × (Avogadro constant)

If each molecule contains n atoms (for example, each H₂O molecule contains n = 3 atoms), then
Total number of atoms = (Total number of molecules) × n

The easiest method is to use Unit Cancellation (Dimensional Analysis).
Or as my the teacher would say "Let the units help work the problem."

If you want molecules, multiply by (6.02 x10e23 Molecules/mole)
but then we need moles on top, so multiply by (moles/gram)

Simply insert your numbers

as Jim Moor wisely said... "use dimensional analysis"

There are countless websites out there that discuss it.. I add my 2 cents in my answer here
https://answers.yahoo.com/question/index?qid=20160219172026AAr3J7B
and my discussion of molar vs molecular vs atomic masses here
https://answers.yahoo.com/question/index?qid=20151203052124AAO9Ulf

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go through those links. then we setup the solution like this

.. ___ g.. .... 1 mol... . 6.022x10^23 molecules (or atoms or whatever)
--- ---- ---- x ---- ---- x --- ---- ---- ----- ----- ---- ---- ---- --- ----- ---- ---- ---- = ___ molecules (atoms or whatever)
.. .. ..1... ... .. ___ g.. ... .... ... .. .... ... ... 1 mol

1 mole ==> Avogadro's number of molecules or atoms (call it A)
so grams to moles
x grms ==> x/ gmole = moles
moles * A = number of atoms/molecues

Multiply by Avogadro's number.