The Foundations of Chemistry

EXAMPLE 2-3 Moles of Atoms

How many moles of atoms does 136.9 g of iron metal contain?

Plan

The atomic weight of iron is 55.85 amu. This tells us that the molar mass of iron is

55.85 g/mol, or that one mole of iron atoms is 55.85 g of iron. We can express this as either

of two unit factors:

or

Because one mole of iron has a mass of 55.85 g, we expect that 136.9 g will be a fairly small

number of moles (greater than 1, but less than 10).

Solution

_?_ mol Fe atoms136.9 g Fe
2.451 mol Fe atoms

You should now work Exercises 34 and 40.

Once the number of moles of atoms of an element is known, the number of atoms in

the sample can be calculated, as Example 2-4 illustrates.

EXAMPLE 2-4 Numbers of Atoms

How many atoms are contained in 2.451 mol of iron?

Plan

One mole of atoms of an element contains Avogadro’s number of atoms, or 6.022
 1023 atoms.

This lets us generate the two unit factors

1 mol Fe atoms



55.85 g Fe

55.85 g Fe



1 mol Fe atoms

1 mol Fe atoms



55.85 g Fe

Figure 2-9 One mole of atoms of
some common elements. Back row
(left to right): bromine, aluminum,
mercury, copper. Front row (left to
right): sulfur, zinc, iron.

60 CHAPTER 2: Chemical Formulas and Composition Stoichiometry

To the required four significant
figures, 1 mol Fe atoms55.85 g Fe.