UNIT 4: THE MOLE & AVOGADRO

SC.912.P.8.9 – Apply the mole concept & Law of Conservation of Mass to calculate the quantities of chemicals

SC.912.P.8.7 – Interpret formula representations of molecules & compounds in terms of composition & structure

VOCABULARY (ADD TO YOUR REFERENCE FOLDER DICTIONARY!)

Molecular formula: a formula that gives the number of atoms of each element present in a molecule. (e.g. H2O, NaCl, C2H6)

Avogadro’s Number: A constant value; The number of “formula units” (atoms, molecules, particles) in one mole of that substance. 6.02 × 1023 = 1 mole (mol)

Atomic Mass Unit (amu): the mass of an atom; found on the Periodic Table.

Molar Mass: The mass (in grams) of 1 mole of a substance; found on the Periodic Table.

Significant Figure Rules: -Add/Subtract: The final calculated answer should have the least # of decimal places

-Multiply/Divide: The final calculated answer should have the least # of overall digits

Q: WHAT IS A MOLE?A: A CHEMIST’S “DOZEN”

Think of the following… How many is 1 dozen?

How many is 1 pair?

In chemistry, we use the same idea!

How many is 1 mole?

1 mole = 6.02 x 1023 “formula units”

Atoms/molecules/particles are too small to individually count for

practical purposes, so we use the term “mole” to count

atoms/molecules/particles more conveniently

Imagine if you had cartons of eggs stacked to the ceiling… what is the easiest/fastest way to count how

many eggs there are? Count 1 egg, 2 eggs, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14… or count by the dozen? 1 dozen,

2 dozen, 3 dozen…

Q: WHAT IS A MOLE?A: A CHEMIST’S “DOZEN”

1 mole of ANYTHING ALWAYS equals 6.02 x 1023 of those things 1 mole of paperclips = 6.02 x 1023 paperclips

1 mole of hydrogen atoms = 6.02 x 1023 hydrogen atoms

1 mole of donuts = 6.02 x 1023 donuts

1 mole of NaCl = 6.02 x 1023 molecules of NaCl

1 mole of Mg(NO3)2 = 6.02 x 1023 molecules of Mg(NO3)2

OKAY SO, HOW DO I KNOW WHEN I HAVE 1 MOLE IF I CAN’T COUNT THE INDIVIDUAL ATOMS/MOLECULES/PARTICLES?

Use the atomic mass found on the Periodic Table!

• Notice that there are no units listed after the atomic mass…

• If you are thinking, “How much does 1 hydrogen atomweigh?” the unit would be a.m.u. (atomic mass unit)… so 1 H atom weighs 1.008 amu. This is called FORMULA MASS

• If you are thinking, “How much does 1 mole of hydrogen weigh?” the unit would be in grams… so 1 mole of H atoms would weigh 1.008 grams. This is called MOLAR MASS• The molar mass of any chemical ALWAYS equals 1 mole of that chemical

CONVERSION FACTORS!

Avogadro’s Number

6.02 x 1023 “formula units” = 1 mole

Molar Mass

Must be calculated using the molecular formula STEPS:1. Get the molecular formula of the molecule2. Determine which elements are in the molecule3. Count how many atoms of each element are in

the molecule4. Find the atomic mass of each element using the

Periodic Table5. Multiply the atomic mass of each element by

however many atoms are in the element6. Add them together! This value, in grams,

equals 1 mole of the molecule

Converts between:

Grams Moles Moles Grams

Converts between:

Formula Units Moles Moles Formula Units

This conversion NEVER CHANGES!

It is a CONSTANT

A NOTE ON COUNTING ATOMS IN A MOLECULAR FORMULA…

MgCl2 What elements are in this molecular formula?

How many of each?

What is the atomic mass of each element, according to the Periodic Table? Multiply the mass by the number of atoms, and add

these together to determine the molar mass (in grams), or formula mass (in a.m.u.)

Mg: 1 x 24.305 = 24.305 Cl: 2 x 35.453 = 70.906 +

95.211 g MgCl2 = 1 mol MgCl2(molar mass)

or the formula mass of MgCl2 is 95.211 a.m.u.

Ca(OH)2

What elements are in this molecular formula?

How many of each?

What is the atomic mass of each element, according to the Periodic Table? Multiply the mass by the number of atoms, and add

these together to determine the molar mass (in grams), or formula mass (in a.m.u.)

Ca: 1 x 40.078 = 40.078 O: 2 x 15.999 = 31.998 H: 2 x 1.008 = 2.016 +

74.092 g Ca(OH)2 = 1 mol Ca(OH)2(molar mass)

or the formula mass of Ca(OH)2 is 74.092 a.m.u.

Distribute the 2 to what is INSIDE the parenthesis by MULTIPLYING, like in

math!

2 TYPES OF FORMULAS…

MOLECULAR FORMULA

A formula that gives the number of atoms of each element present in a molecule. E.g. H2O, NaCl, C2H6

EMPIRICAL FORMULA

A formula that gives the simplest ratio of elements present in a molecule. Instead of C2H6, the empirical formula

is CH3

Instead of C6H12O6, the empirical formula is CH2O

NOTE:If the molecular formula is already in a “reduced” form,

then the empirical formula is the SAME as the molecular formula!

2 TYPES OF FORMULAS…

Search YouTube for: “Writing Empirical Formula Practice Problems” by Tyler DeWitt

“The percent composition of a component (element) in a compound is the percent of the total mass of the compound that is due to that component (element).”

Percent Composition = 𝑀𝑎𝑠𝑠 𝑑𝑢𝑒 𝑡𝑜 𝑎 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑒𝑙𝑒𝑚𝑒𝑛𝑡

𝑇𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑× 100

STEPS:

1. Calculate the molar mass of the compound.

2. Determine how much of that mass is due to a particular component (element) … to put more simply, calculate the molar mass of the particular component (element) in question multiplied by the number of atoms of this element.

3. Divide the mass of the particular component (from step 2) by the molar mass of the compound (from step 1), then multiply this quotient by 100.

PERCENT COMPOSITION

PERCENT COMPOSITION

EXAMPLE: Calculate the percent composition of carbon in CO2.Step 1 – Calculate molar mass of the compound

C: 1 x 12.011 = 12.011

O: 2 x 15.999 = 31.998 +

44.009 g CO2 = 1 mol CO2

Step 2 – Calculate the mass of the molecule DUE TO CARBON

1 x 12.011

Step 3 – Divide step 2 by step 1, then multiply by 100.12.011

44.009× 100 = 𝟐𝟕. 𝟐𝟗𝟐% This means that 27.292% of

CO2 is carbon

Percent Composition = 𝑀𝑎𝑠𝑠 𝑑𝑢𝑒 𝑡𝑜 𝑎 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑒𝑙𝑒𝑚𝑒𝑛𝑡

𝑇𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑× 100

PERCENT COMPOSITION

EXAMPLE: Calculate the percent composition of carbon in C6H12O6

Step 1 – Calculate molar mass of the compound

C: 6 x 12.011 = 72.066

H: 12 x 1.008 = 12.096

O: 6 x 15.999 = 95.994 +

180.156 g C6H12O6 = 1 mol C6H12O6

Step 2 – Calculate the mass of the molecule DUE TO carbon

6 x (12.011 g)

Step 3 – Divide step 2 by step 1, then multiply by 100.

72.066

180.156× 100 = 𝟒𝟎. 𝟎𝟎𝟐%

This means that 40.002% of the mass of C6H12O6 is carbon

Percent Composition = 𝑀𝑎𝑠𝑠 𝑑𝑢𝑒 𝑡𝑜 𝑎 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑒𝑙𝑒𝑚𝑒𝑛𝑡

𝑇𝑜𝑡𝑎𝑙 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑× 100