Section 3.6—Counting Molecules

So the number of molecules affects pressure of an airbag…how do we “count” molecules?

What is a mole?

Definition

Mole – SI unit for counting

The only acceptable abbreviation for “mole” is “mol”…not “m”!!

What is a counting unit?You’re already familiar with one counting unit…a “dozen”

“Dozen” 12

A dozen doughnuts 12 doughnuts

A dozen books

A dozen cars

A dozen people

12 books

12 cars

12 people

A dozen = 12

Why can’t we count atoms in “dozens”?

Atoms and molecules are extremely small

There are 6.02 1023 water molecules in 18mL of water

This means a 12 ounce bottle of water (355 mL) would have 1.19 1025 molecules of water.

That would be 9.89 1023 “dozen” water molecules.

These huge numbers are impractical!

mL355

mL

molecules H2O6.021023

18= _________ molecules H2O1.19 1025

molecules1.19 1025

molecules

dozen1

12= _________ dozen9.89 1023

What does a “mole” count in?

A mole = 6.02 1023 (called Avogadro’s number)

“mole” 6.02 1023

1 mole of doughnuts 6.02 1023 doughnuts

1 mole of atoms

1 mole of molecules

6.02 1023 atoms

6.02 1023 molecules

6.02 1023 = 602,000,000,000,000,000,000,000

This means a 12 ounce bottle of water would have 19.7 “moles” of water…a much easier-to-work-with number!

Example: Molecules & Moles

Example:How many molecules of water

are in 1.25 moles?

= _______ molecules H2O

Example: Molecules & Moles

1.25 mol H2O

mol H2O

Molecules H2O

6.02 1023

17.521023

1 mol = 6.021023 moleculesExample:How many molecules of water

are in 1.25 moles?

Let’s Practice #1

Example:How many moles are equal to 2.8 × 1022 molecules

= _______ moles

Let’s Practice #1

2.8 × 1022 molecules

molecules

mole1

6.02 1023

0.047

1 mol = 6.021023 moleculesExample:How many moles are equal to 2.8 × 1022 molecules

Molar Mass

Definition

Molar Mass – The mass for one mole of an atom or molecule.

Other terms commonly used for the same meaning:Molecular WeightMolecular MassFormula WeightFormula Mass

Mass for 1 mole of atoms

The average atomic mass = grams for 1 mole

Element Mass

1 mole of carbon atoms 12.01 g

1 mole of oxygen atoms

1 mole of hydrogen atoms

16.00 g

1.01 g

Unit for molar mass: g/mole or g/mol

Average atomic mass is found on the periodic table

Molar mass for molecules

The molar mass for a molecule = the sum of the molar masses of all the atoms

Calculating a Molecule’s Mass

Count the number of each type of atom

Find the molar mass of each atom on the periodic table

Multiple the # of atoms molar mass for each atom

Find the sum of all the masses

1

2

3

4

To find the molar mass of a molecule:

Example: Molar Mass

Example:Find the

molar mass for

CaBr2

Example: Molar Mass

Count the number of each type of atom1

Ca

Br

1

2

Example:Find the

molar mass for

CaBr2

Example: Molar Mass

Find the molar mass of each atom on the periodic table2

Ca

Br

1

2

40.08 g/mole

79.91 g/mole

Example:Find the

molar mass for

CaBr2

Example: Molar Mass

Multiple the # of atoms molar mass for each atom3

Ca

Br

1

2

40.08 g/mole

79.91 g/mole

Example:Find the

molar mass for

CaBr2

= 40.08 g/mole

= 159.82 g/mole

Example: Molar Mass

Find the sum of all the masses4

Ca

Br

1

2

40.08 g/mole

79.91 g/mole

= 40.08 g/mole

= 159.82 g/mole+

199.90 g/mole

1 mole of CaBr2 molecules would have a mass of 199.90 g

Example:Find the

molar mass for

CaBr2

Example: Molar Mass & Parenthesis

Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis.

Example:Find the

molar mass for Sr(NO3)2

Example: Molar Mass & Parenthesis

Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis.

1

6

87.62 g/mole

16.00 g/mole

= 87.62 g/mole

= 96.00 g/mole+

211.64 g/mole

1 mole of Sr(NO3)2 molecules would have a mass of 211.64 g

2 14.01 g/mole = 28.02 g/mole

Sr

N

O

Example:Find the

molar mass for Sr(NO3)2

Let’s Practice #2

Example:Find the

molar mass for Al(OH)3

Let’s Practice #2

Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis.

1

3

26.98 g/mole

1.01 g/mole

= 26.98 g/mole

= 3.03 g/mole+

78.01 g/mole

1 mole of Al(OH)3 molecules would have a mass of 78.01 g

3 16.00 g/mole = 48.00 g/mole

Al

O

H

Example:Find the

molar mass for Al(OH)3

Using Molar Mass in Conversions

Example: Moles to Grams

Example:How many grams are

in 1.25 moles of water?

Example: Moles to Grams

1.25 mol H2O = _______ g H2Omol H2O

g H2O18.02

1

22.53

When converting between grams and moles, the molar mass is needed

1 mole H2O molecules = 18.02 g

HO

21

1.01 g/mole16.00 g/mole

= 2.02 g/mole= 16.00 g/mole+

18.02 g/mole

Example:How many grams are

in 1.25 moles of water?

Example: Grams to Molecules

Example:How many molecules

are in 25.5 g NaCl?

25.5 g NaCl

Example: Grams to Molecules

= _________ molecules NaCl

g NaCl

mol NaCl1

58.44

2.63 1023

1 mol = 6.021023 molecules

1 moles NaCl molecules = 58.44 g

NaCl

11

22.99 g/mole35.45 g/mole

= 22.99 g/mole= 35.45 g/mole+

58.44 g/mole

mol NaCl

molecules NaCl6.021023

1

Example:How many molecules

are in 25.5 g NaCl?

Let’s Practice #3

Example:How many moles are in 25.5 g NaCl?

Let’s Practice #3

25.5 g NaCl = _______ mole NaClg NaCl

mole NaCl1

58.44

0.44

1 mole NaCl molecules = 58.44 g

NaCl

11

22.99 g/mole35.45 g/mole

= 22.99 g/mole= 35.45 g/mole+

58.44 g/mole

Example:How many moles are in 25.5 g NaCl?

Let’s Practice #4

Example:How many grams is a sample of 2.75 × 1024

molecules of SrCl2?

2.75 × 1024 molecules SrCl2

Let’s Practice #4

= _________ g SrCl2

molecules SrCl2

mol SrCl21

6.02 × 1023

7.24

1 mol = 6.021023 molecules

1 moles SrCl2 molecules = 158.52 g

SrCl

12

87.62 g/mole35.45 g/mole

= 87.62 g/mole= 70.90 g/mole+

158.52 g/mole

mol SrCl2

g SrCl2158.52

1

Example:How many grams is a sample of 2.75 × 1024

molecules of SrCl2?