Stoichiometry ‹ 89
Moles and Molar Mass
The mole (mol) is the amount of a substance that contains the same number of particles as
atoms in exactly 12 grams of carbon-12. This number of particles (atoms or molecules or
ions) per mole is called Avogadro’s number and is numerically equal to 6.022 × 1023 par-
ticles. The mole is simply a term that represents a certain number of particles, like a dozen
or a pair. That relates moles to the microscopic world, but what about the macroscopic
world? The mole also represents a certain mass of a chemical substance. That mass is the
substance’s atomic or molecular mass expressed in grams. In Chapter 5, the Basics chapter,
we described the atomic mass of an element in terms of atomic mass units (amu). This was
the mass associated with an individual atom. Then we described how one could calculate
the mass of a compound by simply adding together the masses, in amu, of the individual
elements in the compound. This is still the case, but at the macroscopic level the unit of
grams is used to represent the quantity of a mole. Thus, the following relationships apply:
6.022 × 1023 particles = 1 mol
= atomic (molecular, formula) mass in grams
The mass in grams of one mole of a substance is the molar mass.
The relationship above gives a way of converting from grams to moles to particles, and
vice versa. If you have any one of the three quantities, you can calculate the other two.
This becomes extremely useful in working with chemical equations, as we will see later,
because the coefficients in the balanced chemical equation are not only the number of
individual atoms or molecules at the microscopic level, but also the number of moles at the
macroscopic level.
How many moles are present in 1.20 × 1025 silver atoms?
Answer:×
×
(1.20 10 Ag atoms) =
1mol Ag atoms
6.022 10 Ag atoms(^2523) 19.9molAg
Percent Composition and Empirical Formulas
If the formula of a compound is known, it is a fairly straightforward task to determine the
percent composition of each element in the compound. For example, suppose you want
to calculate the percentage of hydrogen and oxygen in water, H 2 O. First calculate the
molecular mass of water:
1 mol H 2 O = 2 mol H + 1 mol O
Substituting the masses involved:
1 mol H 2 O = 2 (1.0079 g/mol) + 16.00 g/mol = 18.0158 g/mol
(intermediate calculation—don’t worry about significant figures yet)
percentage hydrogen = [mass H/mass H 2 O] × 100
= [2(1.0079 g/mol)/18.0158 g/mol] × 100
= 11.19% H
percentage oxygen = [mass O/mass H 2 O] × 100
= [16.00 g/mol/18.0158 g/mol] × 100
= 88.81% O
KEY IDEA