Depends on the quantity.
Assuming that you're dealing with a mass of a substance, the first thing you need to do is determine how many moles are present in that mass.
Once you do that, you use the fact that 1 mole of any substance contains exactly
So, let's say that you want to determine how many molecules are present in 100. g of water.
Use water's molar mass, which tells you what the mass of 1 mole of water is, to determine how many moles you have in 100. g
Now use Avogadro's number to determine how many molecules you have in 5.55 moles
So remember, when having to determine how many molecules or atoms a certain mass contains, always
- use molar mass to determine the number of moles;
- use Avogadro's number to determine the number of molecules/atoms;
Some usefull examples: