Section 3.6—Counting Molecules

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 = 602,000,000,000,000,000,000,000 “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 6.02  1023 formula units 1 mole of formula units 6.02  1023 ions 1 mole of ions

Example: Molecules & Moles Example: How many molecules of water are in 1.25 moles?

Example: Molecules & Moles Example: How many molecules of water are in 1.25 moles? 1 mol = 6.021023 molecules 1.25 mol H2O Molecules H2O 6.02  1023 = _______ molecules H2O 7.521023 1 mol H2O

Let’s Practice #1 Example: How many moles are equal to 2.8 × 1022 molecules of water?

Let’s Practice #1 Example: How many moles are equal to 2.8 × 1022 molecules of water? 1 mol = 6.021023 molecules 1 mole 2.8 × 1022molecules = _______ moles 0.047 6.02  1023 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 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 CaBr2

Example: Molar Mass 1 Count the number of each type of atom Example: Find the molar mass for one formula unit of CaBr2 Ca 1 Br 2

Example: Molar Mass 2 Find the molar mass of each atom on the periodic table Example: Find the molar mass for CaBr2 Ca 1 40.08 g/mole Br 2 79.91 g/mole

Example: Molar Mass 3 Multiple the # of atoms  molar mass for each atom Example: Find the molar mass for CaBr2  Ca 1 40.08 g/mole 40.08 g/mole =  159.82 g/mole Br 2 79.91 g/mole =

Example: Molar Mass 4 Find the sum of all the masses Example: Find the molar mass for CaBr2  Ca 1 40.08 g/mole 40.08 g/mole =  + 159.82 g/mole Br 2 79.91 g/mole = 199.90 g/mole 1 mole of CaBr2 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(NO3)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(NO3)2 Sr  1 87.62 g/mole 87.62 g/mole =  N 2 14.01 g/mole 28.02 g/mole =  + 96.00 g/mole O 6 16.00 g/mole = 211.64 g/mole 1 mole of Sr(NO3)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 =  + 3.03 g/mole H 3 1.01 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?

H 2  1.01 g/mole 2.02 g/mole =  + 16.00 g/mole O 1 16.00 g/mole = 18.02 g/mole 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 mole H2O molecules = 18.02 g 1.25 mol H2O g H2O 18.02 = _______ g H2O 22.5 1 mol H2O

Example: Grams to Formula Units Example: How many formula units are in 25.5 g NaCl?

Na 1  22.99 g/mole 22.99 g/mole =  + 35.45 g/mole Cl 1 35.45 g/mole = 58.44 g/mole Example: Grams to Formula Units Example: How many formula units are in 25.5 g NaCl? 1 moles NaCl f.u.= 58.44 g 1 mol = 6.021023 formula units 25.5 g NaCl mol NaCl molecules NaCl 6.021023 1 58.44 g NaCl 1 mol NaCl 2.63  1023 = _________ formula units NaCl

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: Example: How many liters of Neon at STP are in 3.00 moles of Neon? 1 mole = 22.4 L 3.00 mol Ne L Ne 22.4 = 67.2 L Ne 1 Mol Ne

Section 3.6—Counting Molecules