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108 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 11
First, write the balanced equation.
2Na(s) ϩ Cl 2 (g) 0 2NaCl(s)
Then, use the mole ratio to convert the known number of moles of
chlorine to the number of moles of table salt. Use the formula below.
moles of known ϫ
ϭ moles of unknown
0.02 mol Cl 2 ϫ
ϭ0.04 mol Cl 2
Example Problem 11-2
Stoichiometric Mole-to-Mole Conversion
A piece of magnesium burns in the presence of oxygen, forming
magnesium oxide (MgO). How many moles of oxygen are needed to
produce 12 moles of magnesium oxide?
Write the balanced equation and the mole ratio that relates mol O 2
to mol MgO.
2Mg(s) ϩ O 2 (g) 0 2MgO(s)
Multiply the known number of moles of MgO by the mole ratio.
12 mol MgO ϫ
ϭ6 mol O 2
Six moles of oxygen is needed to produce 12 moles of magnesium
oxide.
Practice Problems
3. The carbon dioxide exhaled by astronauts can be removed from
a spacecraft by reacting it with lithium hydroxide (LiOH). The
reaction is as follows: CO 2 (g) ϩ 2LiOH(s) 0 Li 2 CO 3 (s) ϩ
H 2 O(l). An average person exhales about 20 moles of CO 2 per
day. How many moles of LiOH would be required to maintain
two astronauts in a spacecraft for three days?
1 mol O 2
ᎏᎏ
2 mol MgO
1 mol O 2
ᎏᎏ
2 mol MgO
2 mol NaCl
ᎏᎏ
1 mol Cl 2
moles of unknown
ᎏᎏ
moles of known