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92 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 10
Example Problem 10-2
Converting Number of Particles to Moles
Calculate the number of moles in a sample of sodium bromide
(NaBr) that contains 2.88 ϫ 10 23 formula units.
Because 1 mol NaBr ϭ 6.02 ϫ 10 23 formula units NaBr, you can
see that 2.88 ϫ 10 23 formula units is less than one mole of NaBr.
moles of NaBr ϭ
2.88 ϫ 10 23 formula units ϫ
moles of NaBr ϭ 0.478 mol NaBr
Practice Problems
1. Calculate the number of molecules in 15.7 mol carbon dioxide.
2. Calculate the number of molecules in 0.0544 mol H 2 O.
3. Calculate the number of moles in 9.22 ϫ 10 23 atom iron.
4. Calculate the number of moles of sucrose in a sample that contains 2.05 ϫ 10 22 sucrose molecules.
5. A student uses 0.0850 mol copper sulfate to carry out a reaction. If the reaction uses up 0.0832 mol copper sulfate, how
many formula units are left unreacted?
10.2 Mass and the Mole
One mole of a monatomic element consists of 6.02 ϫ 10 23 atoms of
that element. The mass of a mole of any substance is called the
molar mass of the substance. For example, the molar mass of a
monatomic element is numerically equal to the atomic mass of the
element, but expressed in grams.
molar mass of a substance ϭ
This relationship can be used to convert between mass and moles.
grams of the substance
ᎏᎏᎏ
1 mol of the substance
1 mol NaBr
ᎏᎏᎏ
6.02 ϫ 10 23 formula units