Untitled - Kelly Walsh High School

40 CHEMISTRY FOR THE UTTERLY CONFUSED
3-6 Utterly Confused About
Mole Problems
The mole is the most important concept in this chapter. Nearly every problem
associated with this material requires moles in at least one of the steps. You
should get into the habit of automatically looking for moles. There are several
ways of finding the moles of a substance. You may determine the moles of a
substance from a balanced chemical equation. You may determine moles from
the mass and molecular weight of a substance. You may determine moles from
the number of particles and Avogadro’s number. You may find moles from the
moles of another substance and a mole ratio. Later in this book, you will find
even more ways to determine moles. In some cases, you will be finished when
you find moles, in other cases, finding moles is only one of the steps in a longer
problem.
Let’s use the following chemical equation to examine the different ways of finding
moles:
H 2(g) Cl 2(g) l 2 HCl(g)
It is possible to consider a balanced chemical equation, such as this one, on
many levels. One of these levels is the mole level. At this level, we can get the
moles just by reading the equation in terms of moles. 1 mol of H 2 plus 1 mol of
Cl 2 yields 2 mol of hydrogen chloride.
We can also find the moles of, for example chlorine, from the mass and the
molecular weight. So, if we have 175 g of chlorine, how many moles of chlorine
do we have?
mol Cl2 (175 g Cl2 )a 1 mol Cl2 b 2.4683 2.47 mol Cl2 70.9 g Cl2 The value 70.9 is the molecular weight (mass) of the diatomic chlorine obtained
by multiplying the atomic weight of chlorine (from the periodic table) by two.
If we want to find the moles of a substance from the number of particles, we
need to use Avogadro’s number. For example, let’s find the moles of hydrogen
chloride in 2.62 10 24 molecules of HCl.
mol HCl (2.62 1024 1 mol HCl
molecules HCl)a
6.022 1023 molecules HCl b
4.3507 4.35 mol HCl