# Question #75316

##### 1 Answer

Depends on the quantity.

#### Explanation:

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 **Avogadro's number**.

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:

http://socratic.org/questions/how-many-iron-atoms-are-in-0-550-mol-of-fe2o3