Honors Chemistry
In Class Notes
The Mole: A Measurement of Matter
Measuring Matter

When you complete this presentation, you will be able to:
o describe methods of measuring the amount of something.
o define Avogadro’s number, N0, as it relates to a mole of a substance.
o distinguish between the atomic mass of an element and its molar mass.
o describe how the mass of a mole of compound is calculated.

We live in a world of quantities, of measurement.
o What was your grade on the 1st Semester Exam?
o How many friends do you have on Facebook?
o How big is a “tall” cup of coffee at Starbucks?

Chemists have similar kinds of quantitative questions.
o How many kg of iron can we get from 100 kg of ore?
o How many grams of N2(g) and H2(g) must be combined to give us 200 g of
NH3(g)?
o What volume of CO2(g) is produced when we burn one gallon of gasoline?

To solve chemists’ problems we need to measure the ____________ of matter we have.

What do we mean by the “amount” of matter?

For chemists, this is not a trivial question.

When we measure matter, do we mean …
o the ____________ of atoms or molecules?

How many oxygen molecules combine with each methane molecule
when we burn natural gas?
o the ____________ of the elements or compounds?

How many grams of oxygen (as O2 gas) combines with ten grams of
methane (as CH4 gas) when we burn natural gas?
o the ____________ of the matter?

How many liters of oxygen (as O2 gas) combines with one liter of
methane (as CH4 gas) when we burn natural gas?
1
2
The Mole: A Measurement of Matter

Some units for measuring indicate a specific number of items.
o A ____________ always means two.
o A ____________ always means twelve.

For example, apples are measured in three different ways.
o At a fruit stand, they may be sold by the count (3 for $2.40).
o In a supermarket, you buy apples by the weight ($1.29/lb) or mass
($2.79/kg).
o At an orchard, you buy apples by the volume ($12.00/bushel).

Each of these may be equated to a dozen apples.

For example, apples are measured in three different ways.
o At a fruit stand, they may be sold by the count (3 for $2.40).

By count: 1 dozen apples = 12 apples
o In a supermarket, you buy apples by the weight ($1.29/lb).

By mass: 1dozen apples = 2.0 kg apples
o At an orchard, you buy apples by the volume ($12.00/bushel).


By volume: 1dozen apples = 0.20 bushel apples
Knowing how the count, mass, and volume of apples relates to a dozen apples allows
us to convert between these units.
o For example, we could calculate the mass of …

a bushel of apples or

90 average sized apples
o … by using the following conversion factors
1 dozen apples
12 apples
2.0 kg apples
1 dozen apples
1 dozen apples
0.20 bushel apples
Honors Chemistry
3
In Class Notes
The Mole: A Measurement of Matter
Sample Problem 10.1
What is the mass of 90 average-sized apples if 1 dozen apples has a mass of 2.0 kg?
Known:
# of apples = 90 apples
Unknown:
mass of 90 apples = ? kg
12 apples = 1 dozen apples
1 dozen apples = 2.0 kg
Process:
# of apples  dozens of apples  mass of apples
What is a Mole

Counting things as big as apples is a reasonable way to measure the amount of
something.

Would counting the grains of sand on a beach be a reasonable way to measure the
amount of sand?

Would counting individual atoms in a kilogram of a compound be a reasonable way
to measure the amount of matter?

Counting individual small things (such as atoms or molecules) can be difficult.

It is much easier if the things are grouped into convenient units.
o Counting individual eggs could be tedious.
o Unless they are grouped by dozens.

Chemists use a similar kind of unit to measure amounts of matter (atoms and
molecules).

The unit for the amount of matter is a ____________, abbreviated as ____________.

1 mole of any thing is defined as ________________ pieces of that thing.
o That is 6,020,000,000,000,000,000,000,000 pieces.

This number is called ____________ ____________, ______.
The Mole: A Measurement of Matter

4
Just to be sure you understand this …
o 1 mol of hydrogen atoms is 6.02 × 1024 atoms
o 1 mol of hydrogen molecules is 6.02 × 1024 molecules
o 1 mol of glucose molecules is 6.02 × 1024 molecules
o 1 mol of pencils is 6.02 × 1024 pencils
o 1 mol of students is 6.02 × 1024 students
o 1 mol of grains of sand is 6.02 × 1024 grains

There are only 7.005 × 1020 grains of sand on earth.
o 1 mol of stars is 6.02 × 1024 stars


There are only 9 × 1021 stars in our universe.
Let’s look at a couple of compounds a little more closely.
o 1 mole of hydrogen ____________ is 6.02 × 1023 molecules.

Each hydrogen molecule, H2, is composed of two atoms of ____________.

That means there are 2 × 6.02 × 1023 = 12.04 × 1023 hydrogen
atoms in 1 mole of hydrogen molecules.
o 1 mole of glucose ____________ is 6.02 × 1023 molecules.

Each glucose molecule, C6H12O6, is composed of 6 atoms of ____________,
12 atoms of ____________, and 6 atoms of ____________

That means there are 6 × 6.02 × 1023 = 36.12 × 1023 carbon
atoms in 1 mole of glucose molecules.

That means there are 12 × 6.02 × 1023 = 72.24 × 1023 hydrogen
atoms in 1 mole of glucose molecules.

That means there are 6 × 6.02 × 1023 = 36.12 × 1023 oxygen
atoms in 1 mole of glucose molecules.
Honors Chemistry
5
In Class Notes
The Mole: A Measurement of Matter

Let’s practice converting from number of particles to number of mols.

The conversion looks like this:
# of mol = # of representative particles 

1 mol
6.02 × 1023 representative particles
Let’s try Sample Problem 10.2 (page 291).
Sample Problem 10.2
Magnesium is a light metal used in the manufacture of aircraft, automobile wheels, tools,
and garden furniture. How many moles of magnesium is 1.25 × 1023 atoms of magnesium?
Known:
# of atoms = 1.25 × 1023 atoms
Unknown:
mol = ? mol Mg
1 mol Mg = 6.02 × 1023 atoms Mg
Process:
atoms → mols

Let’s practice converting from number of mols to number of particles .

The conversion looks like this:
# of particles = # of mol 

6.02 × 1023 representative particles
1 mol
Let’s try Sample Problem 10.3 (page 292).
6
The Mole: A Measurement of Matter
Sample Problem 10.3
Propane gas is used for cooking and heating. How many atoms are in 2.12 mol of propane
(C3H8)?
Known:
# of atoms = 1.25 × 1023 atoms
Unknown:
mol = ? mol Mg
1 mol Mg = 6.02 × 1023 atoms Mg
Process:
atoms → mols
The Mass of a Mole

What is so special about Avogadro’s Number?
o Why do we use “__________________” any way?

If we measure out 6.02 × 1023 atoms of an element, the mass is equal to the
____________ ____________ of that element.
o For 6.02 × 1023 atoms of H, mass = 1.01 g
o For 6.02 × 1023 atoms of C, mass = 12.01 g
o For 6.02 × 1023 atoms of O, mass = 16.00 g

If we measure out 6.02 × 1023 molecules of a compound, the mass is equal to the
____________ ____________ of that compound.
o For 6.02 × 1023 molecules of CO2, mass = 44.01 g
o For 6.02 × 1023 molecules of C6H12O6, mass = 180.16 g

The atomic mass of an element is the mass of 1 mol of that element.

The molar mass of a compound is the mass of 1 mol of that compound.
Honors Chemistry

In Class Notes
The Mole: A Measurement of Matter
To calculate the molar mass of a compound
o find the number of grams of each element in one mole of the compound
o add the masses of the elements in the compound

For example:
o for glucose, C6H12O6:

mass of C = 6 × 12.01 g = 72.06 g

mass of H = 12 × 1.00794 g = 12.09528 g

mass of O = 6 × 16.00 g = 96.00 g

mass of C6H12O6 = (72.06 + 12.10 + 96.00) g = 180.16 g
o for water, H2O:

mass of H = 2 × 1.00794 g = 2.01588 g

mass of O = 1 × 16.00 g = 16.00 g

mass of H2O = (12.02 + 16.00) g = 18.02 g
o for methane, CH4:


mass of C = 1 × 12.01 g = 12.01 g

mass of H = 4 × 1.00794 g = 4.03177 g

mass of CH4 = (12.01 + 4.03) g = 16.04 g
Try some on your own:
o find the molar mass of …
NH3
CO2
NaCl
H2SO4
KOH
Fe2O3
7