LESSON
10
What’s in a Mole?
Molar Mass
Think About It
Lead is a highly toxic substance that can be accidentally ingested by humans
and animals. Lead atoms interfere with normal processes in the body causing
disturbances in the nervous system. Suppose you have 100 g of lead (II) carbonate,
PbCO3, and 100 g of lead (II) chloride, PbCl2. It is the lead atoms specifically that
are toxic. If you are exposed to equal masses of lead carbonate and lead chloride,
which substance exposes you to more lead atoms, and is potentially more toxic? To
determine this, you need to determine the mass of 1 mol of each.
How can you convert between mass and moles?
To answer this question, you will explore
Molar Mass of Compounds
Comparing a Mole’s Worth
Exploring the Topic
Molar Mass of Compounds
Counting With Moles
Any object can be counted with units such as a dozen or a million or a mole.
Consider a dozen cheese sandwiches. Each sandwich has two slices of bread and
one slice of cheese. Therefore, a dozen sandwiches would have a total of two
dozen slices of bread and one dozen slices of cheese.
In a similar way, you can count atoms of lead in a lead compound. Because a
compound consists of bonded atoms, you can simply add the molar masses of
each atom in a molecule or formula unit of the compound to obtain the molar
mass of the substance. For example, each formula unit of lead (II) chloride, PbCl2,
contains one atom of lead and two atoms of chlorine.
Molar mass of lead (II) chloride, PbCl2
1 lead atom
1 mol lead atoms
1 lead (II) chloride
formula unit
1 mol lead (II) chloride
2 mol chlorine atoms
2 chlorine atoms
The molar masses of lead and chlorine are 270.21 g/mol and 35.45 g/mol,
respectively.
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H E A LT H
CO N N E C T I O N
Lead (II) carbonate is one
of the white pigments in
lead paint. Lead paint
dries quickly and forms a
durable shiny coating. It
is still used to draw lines
on pavement, but it is
no longer used to paint
houses or furniture due to
the high toxicity of lead.
Adding the molar mass of lead and twice the molar mass of chlorine gives the molar
mass of lead chloride, PbCl2, which is 341.11 grams per mole, or 341.11 g/mol.
Example 1
The Mass of One Mole
Suppose you have 1 mol of lead, Pb, 1 mol of lead (II) chloride, PbCl2, and 1 mol
of lead (II) carbonate, PbCO3. What is the mass of each sample?
Solution
You can find the molar mass of lead, Pb, on the periodic table, 207.2 g/mol.
A mole of PbCl2 has 1 mol of lead atoms and 2 mol of chlorine atoms. You find
the molar mass of the compound by adding these molar masses together.
molar mass of PbCl2 207.2 2(35.45)
278.1 g/mol
So the mass of 1 mol of lead (II) chloride is 276.1 g. The molar mass of PbCO3 can
be found similarly.
molar mass of PbCO3 207.2 12.01 3(16.00)
267.2 g/mol
So the mass of 1 mol of lead (II) carbonate is 267.2 g.
[For a review of this math topic, see MATH Spotlight: Solving Equations
on page 620.]
Comparing a Mole’s Worth
A mole of atoms or molecules is usually an amount that you can hold in your
hand if the substance is liquid or solid. A table with a “mole’s worth” of a few
substances is given below.
Molar Masses
Chemical
formula
Molar mass
(g /mol)
Moles of what?
Equivalent to
O2 (g)
31.98 g/mol
oxygen molecules
22.4 L oxygen gas
Al(s)
26.98 g/mol
Al atoms
2 aluminum cans
H2O(l)
18.01 g/mol
H2O molecules
18 mL water
He(g)
4.00 g/mol
He atoms
22.4 L helium gas
NaCl(s)
58.44 g/mol
sodium chloride
units
1/4 cup salt
C12H22O11(s)
342.23 g/mol
sugar molecules
0.75 lb sugar
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Note that the molar mass of oxygen gas, O2 (g), is double the molar mass of
oxygen found on the periodic table because oxygen gas is diatomic. Also note that
the volume of a mole of O2 gas is the same as the volume of a mole of any other
gas at standard temperature and pressure, or STP.
Example 2
Toxicity of Lead Compounds
Which is potentially the most toxic: 1 g lead, Pb, 1 g lead (II) chloride, PbCl2, or
1 g lead (II) carbonate, PbCO3?
Solution
The molar mass of each compound was calculated in Example 1.
molar mass of Pb 207.2 g/mol
molar mass of PbCl2 278.1 g/mol
molar mass of PbCO3 267.21 g/mol
Because Pb has the lowest molar mass, 1 g lead, Pb, will have the largest number
of moles of lead, so it will potentially be the most toxic, followed by lead (II)
chloride, PbCl2.
Lesson Summary
How can you convert between mass and moles?
Chemists compare moles of substances rather than masses of substances because
moles are a way of counting atoms, molecules, or units in a compound. The molar
mass of a substance is the mass, in grams, of one mole of the substance. The molar
mass of a compound is the sum of the molar masses of the atoms in the compound.
EXERCISES
Reading Questions
1. Explain how to determine the molar mass of sodium chloride, NaCl.
2. Describe the approximate size of 1 mol of a solid, a liquid, and a gas. Give
a specific example of each.
M AT H E M AT I C S
CO N N E C T I O N
In daily life you would never use the mole as a counting
unit. It is just too big. For example, there is nowhere
near a mole of pennies in the world, sand grains
on Earth, or stars in the known universe. In other
words, there are more atoms of carbon in a pencil
than there are grains of sand on Earth or stars in the
known universe!
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Reason and Apply
3. Copy this table and use a periodic table to complete the second column
Chemical formula
Molar mass (g /mol)
Moles of what?
Ne(g)
1 mol Ne atoms
Ca(s)
1 mol Ca atoms
CO2 (g)
1 mol carbon dioxide molecules
CaCO3 (s)
1 mol calcium carbonate units
CH4O(l)
1 mol methanol molecules
C2H6O(l)
1 mol ethanol molecules
Fe2O3 (s)
1 mol iron oxide units
4. Which has more moles of molecules, 1.0 g methanol, CH4O, or
1.0 g ethanol, C2H6O?
5. Which has more moles of metal atoms?
a. 10.0 g calcium, Ca, or 10.0 g calcium chloride, CaCl2
b. 5.0 g sodium chloride, NaCl, or 5.0 g sodium fluoride, NaF
c. 2.0 g iron oxide, FeO, or 2.0 g iron sulfide, FeS
6. How many grams of carbon molecules are in 1 mol of each substance?
a. methane, CH4
b. methanol, CH4O
c. ethanol, C2H6O
7. What is the mass of 5 mol of iron (III) oxide, Fe2O3?
8. Which of these has the most chromium?
A. 1.0 g chromium (II) chloride, CrCl2
B. 1.0 g chromium (III) chloride, CrCl3
C. 1.0 g chromium (IV) oxide, CrO2
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