Section 3 6Counting Molecules So the number of

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 • Abbreviated as “mol”

What is a counting unit? You’re already familiar with one counting unit…a “dozen” A dozen = 12 “Dozen” 12 A dozen doughnuts 12 doughnuts A dozen books 12 books A dozen cars 12 cars A dozen people 12 people

What are we counting in Chemistry? In Chemistry we count Representative Particles! What is a REPRESENTATIVE PARTICLE? • The smallest piece of matter that still represents that substance • Examples of representative particles • The smallest piece of an element = ATOM • Example: I have one ATOM of Copper • The smallest piece of a covalent compound = MOLECULE • Example: I have one molecule of water • The smallest piece of an ionic compound = FORMULA UNIT • Example: I have one formula unit of sodium chloride

What does a “mole” count by? A mole = 6. 02 1023 (called Avogadro’s number) 6. 02 1023 = 602, 000, 000, 000 “mole” 6. 02 1023 1 mole of doughnuts 6. 02 1023 doughnuts 1 mole of atoms 6. 02 1023 atoms 1 mole of molecules 6. 02 1023 molecules 1 mole of formula units 6. 02 1023 formula units 1 mole of ions 6. 02 1023 ions

Example: Molecules & Moles Example: How many molecules of water are in 1. 25 moles?

Example: Molecules & Moles 1 mol = 6. 02 1023 molecules Example: How many molecules of water are in 1. 25 moles? 1. 25 mol H 2 O 6. 02 1023 1 Molecules H 2 O mol H 2 O 7. 52 1023 molecules H 2 O = _______

Let’s Practice #1 Example: How many moles are equal to 2. 8 × 1022 molecules of water?

Let’s Practice #1 1 mol = 6. 02 1023 molecules Example: How many moles are equal to 2. 8 × 1022 molecules of water? 2. 8 × 1022 molecules 1 mole 6. 02 1023 molecules 0. 047 = _______ moles

Molar Mass

Definition Molar Mass – The mass for one mole of an atom or molecule. Other terms commonly used for the same meaning: Molecular Weight Molecular Mass Formula Weight Formula Mass

Mass for 1 mole of atoms The average atomic mass = grams for 1 mole Average atomic mass is found on the periodic table Element Mass 1 mole of carbon atoms 12. 01 g 1 mole of oxygen atoms 16. 00 g 1 mole of hydrogen atoms 1. 01 g Unit for molar mass: g/mole or g/mol

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 To find the molar mass of a molecule: 1 Count the number of each type of atom 2 Find the molar mass of each atom on the periodic table 3 Multiple the # of atoms molar mass for each atom 4 Find the sum of all the masses

Example: Molar Mass Example: Find the molar mass for one formula unit of Ca. Br 2

Example: Molar Mass 1 Example: Find the molar mass for one formula unit of Ca. Br 2 Count the number of each type of atom Ca 1 Br 2

Example: Molar Mass 2 Example: Find the molar mass for Ca. Br 2 Find the molar mass of each atom on the periodic table Ca 1 40. 08 g/mole Br 2 79. 91 g/mole

Example: Molar Mass 3 Example: Find the molar mass for Ca. Br 2 Multiple the # of atoms molar mass for each atom Ca 1 40. 08 g/mole = 40. 08 g/mole Br 2 79. 91 g/mole = 159. 82 g/mole

Example: Molar Mass 4 Example: Find the molar mass for Ca. Br 2 Find the sum of all the masses Ca 1 40. 08 g/mole = 40. 08 g/mole Br 2 79. 91 g/mole = + 159. 82 g/mole 199. 90 g/mole 1 mole of Ca. Br 2 molecules would have a mass of 199. 90 g

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(NO 3)2

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(NO 3)2 Sr 1 87. 62 g/mole = 87. 62 g/mole N 2 14. 01 g/mole = 28. 02 g/mole O 6 16. 00 g/mole = + 96. 00 g/mole 211. 64 g/mole 1 mole of Sr(NO 3)2 molecules would have a mass of 211. 64 g

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. Example: Find the molar mass for Al(OH)3 Al 1 26. 98 g/mole = 26. 98 g/mole O 3 16. 00 g/mole = 48. 00 g/mole H 3 1. 01 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

Using Molar Mass in Conversions STOP – Teacher will return to this topic after Thanksgiving break!

A Handy Mole Conversion Chart! This chart shows how you can start in ANY oval and convert to any other oval. Ex. If I have 5 grams of Na but I needed the total number of particles – I would start in the mass oval and go first to the mole and then to the number of particles! Clearly a 2 step process

Example: Moles to Grams Example: How many grams are in 1. 25 moles of water?

Example: Moles to Grams When converting between grams and moles, the molar mass is needed Example: How many grams are in 1. 25 moles of water? 1. 25 mol H 2 O H 2 1. 01 g/mole = 2. 02 g/mole O 1 16. 00 g/mole = + 16. 00 g/mole 18. 02 g/mole 1 mole H 2 O molecules = 18. 02 g H 2 O 1 mol H 2 O 22. 5 = _______ g H 2 O

Example: Grams to Formula Units Example: How many formula units are in 25. 5 g Na. Cl?

Example: Grams to Formula Units Example: How many formula units are in 25. 5 g Na. Cl? Na 1 22. 99 g/mole = 22. 99 g/mole Cl 1 35. 45 g/mole = + 35. 45 g/mole 58. 44 g/mole 1 moles Na. Cl f. u. = 58. 44 g 1 mol = 6. 02 1023 formula units 25. 5 g Na. Cl 1 mol Na. Cl 58. 44 g Na. Cl 6. 02 1023 molecules Na. Cl 1 mol Na. Cl 2. 63 1023 formula units Na. Cl = _____

Volume and the Mole! For any gas that is at standard temperature and pressure, the following relationship is true: 1 mole of any gas will occupy 22. 4 Liters of space 1 mole = 22. 4 L @STP

What is STP? ? ? SO Easy! Standard Temperature and Pressure means that the gas is at zero degrees Celsius and one atmosphere of pressure! Two easy numbers to memorize! O and 1

Example Using Volume Example: How many liters of Neon at STP are in 3. 00 moles of Neon? 3. 00 mol Ne 1 mole = 22. 4 L Ne 1 Mol Ne = 67. 2 L Ne