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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
91
The Mole
The Mole
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 10
10.1 Measuring Matter
Substances react according to definite ratios of numbers of particles
(atoms, ions, formula units, or molecules). The following balanced
chemical equation shows that two atoms of aluminum react with
three molecules of iodine to form two formula units of aluminum
iodide.
2Al(s) ϩ 3I 2 (s) 0 2AlI 3 (s)
If you wanted to carry out this reaction, how could you measure the
correct numbers of particles of aluminum and iodine? Fortunately,
you can count particles by measuring mass. Suppose you have a
sample of an element, and the mass of the sample in grams is
numerically equal to the atomic mass of the element. Scientists have
discovered that this mass of an element contains 6.02 ϫ 10 23 atoms
of that element. This number is called Avogadro’s number.
Avogadro’s number of particles is called a mole of particles. The
mole is the SI base unit used to measure the amount of a substance
and is defined as the number of particles in exactly 12 g of pure
carbon-12. For the purpose of dealing with moles, the simplest unit
of any substance is called a representative particle. The particle may
be an atom, a molecule, a formula unit, or an ion.
Example Problem 10-1
Converting Moles to Number of Particles
How many molecules are in 2.25 moles of bromine (Br 2 )?
Bromine is an element that consists of diatomic molecules.
Therefore, one mole of bromine contains 6.02 ϫ 10 23 Br 2 molecules.
To find the number of Br 2 molecules present in 2.25 mol, multiply
the number of moles by Avogadro’s number.
number of Br 2 molecules ϭ
2.25 mol ϫ
number of Br 2 molecules ϭ 1.35 ϫ 10 24 molecules Br 2
6.02 ϫ 10 23 molecules Br 2
ᎏᎏᎏ
1 mol
Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
91
The Mole
The Mole
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 10
10.1 Measuring Matter
Substances react according to definite ratios of numbers of particles
(atoms, ions, formula units, or molecules). The following balanced
chemical equation shows that two atoms of aluminum react with
three molecules of iodine to form two formula units of aluminum
iodide.
2Al(s) ϩ 3I 2 (s) 0 2AlI 3 (s)
If you wanted to carry out this reaction, how could you measure the
correct numbers of particles of aluminum and iodine? Fortunately,
you can count particles by measuring mass. Suppose you have a
sample of an element, and the mass of the sample in grams is
numerically equal to the atomic mass of the element. Scientists have
discovered that this mass of an element contains 6.02 ϫ 10 23 atoms
of that element. This number is called Avogadro’s number.
Avogadro’s number of particles is called a mole of particles. The
mole is the SI base unit used to measure the amount of a substance
and is defined as the number of particles in exactly 12 g of pure
carbon-12. For the purpose of dealing with moles, the simplest unit
of any substance is called a representative particle. The particle may
be an atom, a molecule, a formula unit, or an ion.
Example Problem 10-1
Converting Moles to Number of Particles
How many molecules are in 2.25 moles of bromine (Br 2 )?
Bromine is an element that consists of diatomic molecules.
Therefore, one mole of bromine contains 6.02 ϫ 10 23 Br 2 molecules.
To find the number of Br 2 molecules present in 2.25 mol, multiply
the number of moles by Avogadro’s number.
number of Br 2 molecules ϭ
2.25 mol ϫ
number of Br 2 molecules ϭ 1.35 ϫ 10 24 molecules Br 2
6.02 ϫ 10 23 molecules Br 2
ᎏᎏᎏ
1 mol