Unit 5 Section 1: The Mole Concept

Unit 5 Section 1: The Mole Concept

Lesson Objectives

• Review Ideas for converting between two units

• Understand the concept of a mole

• Relate Moles to Grams and molar mass

• Calculating the Molar Mass of Compounds

• Converting between moles and Various Units

Measuring Matter

• Chemistry is a quantitative science where we keep track of exact amounts of matter

• There are three ways to quantify matter that we can convert between

  • count

  • mass

  • volume

Measuring Matter

• Lets use apples as an example

  • We can count how many apples we have

    • 12 apples = 1 dozen

  • We can weigh our apples

    • 12 apples weigh ~ 2.0 kg

  • We can measure how much space our apples take up

    • 12 apples fills 0.20 bushels

Measuring Matter Sample problem 1

• If 12 apples fills 0.2 Bushels, How many bushels do we have if we have 50 apples?

  • This works just like our conversion factors!

Multiple Choice

What is the the conversion factor we want to use to convert from apples to bushels?

• 12 apples = 1 bushel

• 0.2 apples = 12 bushels

• 12 apples = 0.2 bushels

• 1 apple = 1 bushel

Measuring Matter Sample problem 1

• If 12 apples fills 0.2 Bushels, How many bushels do we have if we have 50 apples?

  • This works just like our conversion factors!

    • 12 apples = 0.2 bushels

  • We can use our relationship to convert our 50 apples into bushels

Measuring Matter Sample problem 2

• What is the mass of 90 apples if 1 dozen apples have a mass of 2.0 kg

  • what relationships do we know?

    • 1 dozen apples have a mass of 2.0 kg

    • How much is a dozen?

Measuring Matter Sample problem 2

• What is the mass of 90 apples if 1 dozen apples have a mass of 2.0 kg

  • what relationships do we know?

    • 1 dozen apples have a mass of 2.0 kg

    • 1 dozen apples = 12 apples

  • Step 1: Convert our apples to dozens

  • Step 2 Convert our dozens to kg

Counting with moles

• In chemistry, we work with tiny things that are hard to count individually, so we group them into a unit called a mole

  • A mole is similar to a dozen, where it is a quick way of expressing the amount you have one object

• 1 mole= 6.02 × 10²³ things

  • Anything can be counted in moles, but in chemistry, we count particles, atoms, and formula units

Why 6.02 × 10²³ ?

• 6.02 × 10²³ is called Avogadro's number

  • Named for an Italian Scientist who made many contributions to chemistry

Converting the number of Atoms to Moles Sample Problem 1

• How many moles of magnesium do you have when you have 1.25 × 10²³ atoms of magnesium?

  • Step 1: Identify your known relationship

    • 1 Mole Mg = 6.02 × 10²³ atoms

  • Step two Set up your conversion factor

Converting the number of Atoms to Moles Sample Problem 2

• How many moles of Bromine do you have when you have 2.17 × 10²³ particles?

  • Step 1: Identify your known relationship

    • 1 Mole Br = 6.02 × 10²³ atoms

  • Step two Set up your conversion factor

Molar Mass

• When we measure things like molecules, they can contain different numbers of atoms and be of different sizes

  • NaCl has 1 Sodium and 1 Chlorine = 2 atoms total

  • H₂O has 2 hydrogen and 1 oxygen= 3 atoms total

• The mass of one mole of any atom/molecule will be dependent on how big the atom/molecule is

• Molar Mass: The mass of one mole of a substance

  • For elements, this is the same as the atomic mass, with the unit being grams/mol

Match

Match the following elements to their Rounded molar masses

• H

• O

• C

• N

• Cu

1

16

12

14

64

• 1

• 16

• 12

• 14

• 64

Multiple Choice

What is the correct unit for molar mass?

• gram

• g/mol

• liter

• mole

Dropdown

One ​

Select answer

is equal to 6.02 ×​ 10²³, This number is also called​

Select answer

Molar Mass of Compounds

• Each element has its own molar mass

• When you have a compound made up of multiple elements, you simply add the molar masses for the total amounts of elements together

Molar Mass of Compounds Sample Problem 1

• What is the mass of Hydrogen Peroxide (H₂O₂) rounded to a whole number?

  • Step 1: Identify the amounts we have of each element

    • 2 hydrogen

    • 2 oxygen

  • Step 2: Identify the mass of all relevant atoms on the periodic table

    • Hydrogen = 1 g/mol

    • Oxygen= 16 g/mol

  • Step 3: Find our total mass by multiplying the number of each atom by its mass

    • (2 × 1) + (2 × 16) = 2 + 32

Molar Mass of Compounds Sample Problem 2

• Phosphorous trichloride PCl₃ is used in the production of many herbicides and pesticides. What is the Molar Mass of (PCl₃)

  • Step 1: Identify the amounts we have of each element

Dropdown

Phosphorous Trichloride (PCl₃) has​

Select answer

phosphorous atom and ​

Select answer

chlorine atoms

Molar Mass of Compounds Sample Problem 2

• Phosphorous trichloride PCl₃ is used in the production of many herbicides and pesticides. What is the Molar Mass of (PCl₃)

  • Step 1: Identify the amounts we have of each element

    • 1 phosphorous

    • 3 Chlorine

  • Step 2: Identify the mass of all relevant atoms on the periodic table

Multiple Choice

What is the Molar mass of phosphorous rounded to a whole number?

• 31

• 15

• 17

• 35

Multiple Choice

What is the Molar mass of chlorine rounded to a whole number?

• 31

• 15

• 17

• 35

Molar Mass of Compounds Sample Problem 2

• Phosphorous trichloride PCl₃ is used in the production of many herbicides and pesticides. What is the Molar Mass of (PCl₃)?

  • Step 1: Identify the amounts we have of each element

    • 1 phosphorous

    • 3 Chlorine

  • Step 2: Identify the mass of all relevant atoms on the periodic table

    • Phosphorous = 31 g/mol

    • Chlorine= 35 g/mol

  • Step 3: Find our total mass by multiplying the number of each atom by its mass

    • (1 × 31) + (3 × 35)

Avogadro's Hypothesis

• When we look at the volume of gases, we use the unit liters as our bas unit

• All Gases have the same molar volume at STP, regardless of their size

  • 1 mole=22.4 L

Molar Conversions

• Sometimes we want to change from particles to grams or grams to liters

• When we do this, we use Moles as an intermediate step for all substances

Converting With Moles sample Problem 1

• Aluminum combines with oxygen to create aluminum oxide (Al₂O₃), which protects metal from corrosion. What is the mass in grams of 9.45 mol of aluminum oxide?

  • Step 1: Identify what we need to know

Multiple Choice

To find out the mass of 9.45 moles of Al₂O₃, what do we need to know

• the molar mass of Al₂O₃

• The liters of Al₂O₃

• The number of particles of Al₂O₃

• nothing, the moles and mass are the same thing

Converting With Moles sample Problem

• Aluminum combines with oxygen to create aluminum oxide (Al₂O₃), which protects metal from corrosion. What is the mass in grams of 9.45 mol of aluminum oxide?

  • Step 1: Identify what we need to know

    • The molar mass of Al₂O₃

  • Step 2: Calculate the molar mass of Al₂O₃

    • 2 Aluminum, 3 Oxygen

Multiple Choice

What is the rounded molar mass of Aluminum?

• 8

• 16

• 13

• 27

Multiple Choice

What is the molar mass of Oxygen

• 8

• 16

• 13

• 27

Converting With Moles sample Problem 1

• Aluminum combines with oxygen to create aluminum oxide (Al₂O₃), which protects metal from corrosion. What is the mass in grams of 9.45 mol of aluminum oxide?

  • Step 1: Identify what we need to know

    • The molar mass of Al₂O₃

  • Step 2: Calculate the molar mass of Al₂O₃

    • 2 Aluminum, 3 Oxygen

    • 1 Aluminum=27, 1 Oxygen= 16

    • (2 × 27) + (3×16)

Converting With Moles sample Problem 1

• Aluminum combines with oxygen to create aluminum oxide (Al₂O₃), which protects metal from corrosion. What is the mass in grams of 9.45 mol of aluminum oxide?

  • Step 1: Identify what we need to know

    • The molar mass of Al₂O₃

  • Step 2: Calculate the molar mass of Al₂O₃

    • Al₂O₃ = 102 g/mol

  • Step 3: Convert your moles to grams

Converting With Moles Sample Problem 2

• What is the mass in grams of 2.50 mol of Oxygen Gas (O₂)

  • Step 1: Identify what you need to know

    • The Molar mass of O₂

Converting With Moles Sample Problem 2

• What is the mass in grams of 2.50 mol of Oxygen Gas (O₂)

  • Step 1: Identify what you need to know

    • The Molar mass of O₂

    • Oxygen has a molar mass of 16. O₂ has 2 oxygen

      • 2 × 16 =32 g/mol

  • Step 2: Use your molar mass as a conversion factor for your wanted amount