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Mass, Moles, and Equations 41
Careful!
Many students use Avogadro’s number in inappropriate places. You should only
use Avogadro’s number if you have or need the number of particles.
Let’s examine the last of the common ways to find moles. This method requires
that you have the moles of one substance in order to get the moles of another
substance. The moles of the first substance may be given to you or you may
have to use one of the above methods to determine the initial mole value.
Suppose we wished to know the number of moles of HCl that we might form
from 2.2 mol of H 2.
mol HCl (2.2 mol H 2 )a
2 mol HCl
b 4.4 mol HCl
1 mol H2 The term (2 mol HCl/1 mol H 2) is a mole ratio. We got this mole ratio directly
from the balanced chemical equation. The balanced chemical equation has a 2
in front of the HCl, thus we have the same number in front of the “mol HCl.”
The balanced chemical equation has an understood 1 in front of the H 2, for this
reason the same value belongs in front of the “mol H 2.” The values in the mole
ratio are exact numbers, and, as such, do not affect the significant figures.
Let’s use the information from this chapter in one problem. This will be a long
problem that will probably have more facets to it than any other problem you
may see, but it will help you to see all sides of mole problems. We will begin by
stating the problem:
Solid iodine, I 2, will react with fluorine gas, F 2 , to form gaseous iodine pentafluoride,
IF 5. In one experiment, a scientist mixed 75.0 g of iodine with 4.00 10 23
molecules of fluorine and allowed them to react. What is the percent yield if the
reaction produced 45.2 g of iodine pentafluoride?
There are many important aspects of this problem. We must begin by extracting
the information. We have enough information to begin writing a reaction
and several quantities (with associated units).
I2(s) F2(g) l IF5(g) (unbalanced)
75.0 g 4.00 1023 molecules 45.2 g
? percent yield