Chemistry Reference
In-Depth Information
Lesson 7-2: Molar Conversions
Now that you understand the mole, this lesson will show you how you
can use this knowledge to analyze real-life samples in the laboratory. For
example, you can figure out how many atoms you have in a sample of alu-
minum that you mass or in a volume of hydrogen gas you collect in the lab.
Some students resort to “tricks” or mechanical aids, such as the “mole map”
that is common in many classrooms. Although these aids, and the mole
map in particular, can be very helpful, they stress mechanical problem solv-
ing over logical problem solving. I would caution you not to try to replace
understanding with these aids. Rather, try to truly understand each ques-
tion and solve it logically. “Tricks” tend to have very limited applications,
but good problem-solving techniques will serve you well in unlimited ways.
Changing the Mass to Number of Moles
Suppose your instructor gave you a piece of pure aluminum foil and
asked you to determine how many atoms were in the sample. Would you
know how to go about solving the problem? What information do you have
to start with? What other information could you easily determine in your
laboratory setting?
To start with, you could find the mass of your aluminum by placing it
on your laboratory balance. Let's say, for the sake of argument, that the
mass of your sample was 60.0 g. You could then find the molar mass of
aluminum by simply looking at the periodic table. The table tells us that
the atomic mass of aluminum is 27.0 u/atom. Therefore the molar mass of
aluminum is 27.0 g/mole (as explained in Lesson 7-1).
Let's summarize what we know:
The mass of the sample of aluminum = 60.0 g
The molar mass of aluminum is 27.0 g/mole
Do we have more than one mole of aluminum or less than one mole of
aluminum? Remember: One mole of aluminum has a mass of 27.0 grams,
and you have 60.0 grams. So clearly you have more than one mole of alu-
minum. Exactly how many moles of aluminum do you have?
If you don't see how to solve this right away, try thinking in terms of
money. If you went to a store with $60.00 and you wanted to buy shirts that
cost $27.00, how many shirts could you buy? You probably figured that
(assuming there isn't too much tax) you could buy two shirts. Analyze how
