Chemistry Reference
In-Depth Information
you solved the problem. If you had to write an equation for figuring out
how many shirts you could buy, it might look like the following:
total money you have
cost of 1 shirt
$ 60.00
$ 27.00/shirt
# of shirts you can buy =
=
= 2.22 shirts
Of course, we didn't include tax, which varies from place to place, and
the store probably won't sell you a fraction of a shirt. Moles, however, do
come in fractions. Using this model, let's see how we can solve our original
mole problem.
Example 1
How many moles does 60.0 g of aluminum represent?
# of moles of a substance =
total mass of the sample
molar mass of substance
60.0 g
27.0 g/mole
= 2.22 moles
=
As you can see, making this type of conversion is quite easy when deal-
ing with elements. Dealing with compounds is only slightly harder, as illus-
trated in our next example.
Suppose you need to find the number of moles represented by 80.0
grams of water. In this problem, the only additional step would be adding
up the molar masses (you can review this topic in Chapter 5) of the ele-
ments that make up the water, as shown here:
The molar mass of water (H
2
O) is:
Hydrogen (H
2
) = 1.01 g/mole × 2 mole = 2.02 g
Oxygen (O) = 16.0 g/mole × 1 mole = + 16.0 g
Molar mass of water = 18.0 g
Now, we can go about solving the problem in the same way we solved
Example 1.
Example 2
How many moles does 80.0 g of water represent?
total mass of the sample
molar mass of substance
80.0 g
18.0 g/mole
= 4.44 moles
# of moles of a substance =
=
Changing the Number of Moles to Mass
Now, let's look at a slightly different scenario. Let's suppose your instruc-
tor told you that you would need 0.750 moles of CuCl
2
to begin a reaction in
