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Section 3.6—Counting Molecules

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1 Section 3.6—Counting Molecules
So the number of molecules affects pressure of an airbag…how do we “count” molecules?

2 What is a mole?

3 Definition Mole – SI unit for counting Abbreviated as “mol”

4 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

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6 Watch the mole video from siemianowski.weebly.com (best website ever)

7 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

8 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 1 mole of formula units 6.02  1023 formula units 1 mole of ions 6.02  1023 ions

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

10 Example: Molecules & Moles
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

11 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?

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

13 Molar Mass What is molar mass?

14 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

15 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

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

18 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

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21 Find the molar mass for one formula unit of CaBr2
Example: Molar Mass Example: Find the molar mass for one formula unit of CaBr2

22 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

23 Find the molar mass for CaBr2
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

24 Find the molar mass for CaBr2
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 Br 2 79.91 g/mole = g/mole

25 Find the molar mass for CaBr2
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 Br 2 79.91 g/mole = + g/mole g/mole 1 mole of CaBr2 molecules would have a mass of g

26 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

27 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 O 6 16.00 g/mole = + 96.00 g/mole g/mole 1 mole of Sr(NO3)2 molecules would have a mass of g

28 Find the molar mass for Al(OH)3
Let’s Practice #2 Example: Find the molar mass for Al(OH)3

29 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 g

30 Using Molar Mass in Conversions
What is molar mass?

31 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 

32 Gasses are weird…

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34 Example: Moles to Grams
How many grams are in 1.25 moles of water?

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

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

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

38 1 mole of any gas will occupy 22.4 Liters of space
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

39 Two easy numbers to memorize! O and 1
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

40 How many liters of Neon at STP are in
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 22.4 L Ne = L Ne 1 Mol Ne

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42 Welcome to Mole Island 1 mol = molar mass 1 mole = 22.4 L @ STP
6.02 x 1023 particles

43 Stoichiometry Island Diagram
Known Unknown Substance A Substance B M Mass Mass Mountain Mass Mole Island Volume Mole Mole Volume V Liter Lagoon Particles Particles P Particle Place Stoichiometry Island Diagram

44 Stoichiometry Island Diagram
Known Unknown Substance A Substance B M Mass Mass Mountain Mass Volume Mole Mole Volume V Liter Lagoon Particles Particles P Particle Place Stoichiometry Island Diagram

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46 Stoichiometry Mole Island Diagram When in doubt…convert to moles!
Known Unknown Substance A Substance B Mass Mass 1 mole = molar mass (g) 1 mole = molar mass (g) Use coefficients from balanced chemical equation Volume Mole Mole Volume 1 mole = STP 1 mole = STP (gases) (gases) 1 mole = x 1023 particles (atoms or molecules) 1 mole = x 1023 particles (atoms or molecules) Particles Particles

47 A. Using Mole Island Zn + 2HCl  ZnCl2 + H2 79.1 g ? L 79.1 g Zn 1 mol
Just watch, don’t write: If you have 79.1 g of Zn to react with excess HCl (enough HCl to keep reacting as long as you need), how many Liters of H2 gas will be produced? Zn HCl  ZnCl H2 79.1 g ? L 79.1 g Zn 1 mol Zn 65.39 g Zn 1 mol H2 Zn 22.4 L H2 1 mol = 27.1 L H2

48 B. Using Mole Island Zn + 2HCl  ZnCl2 + H2 ? L 79.1 g 79.1g HCl 1 mol
Now write this one for practice: If you instead have 79.1 g of HCl to react with excess Zinc (enough Zinc to keep reacting as long as you need), how many Liters of H2 gas will be produced? Zn HCl  ZnCl H2 ? L 79.1 g 79.1g HCl 1 mol HCl 36.5g HCl 1 mol H2 2 mol HCl 22.4 L H2 1 mol = 24.3 L H2

49 Zn + 2HCl  ZnCl2 + H2 79.1 g 79.1 g HCl 1 mol HCl 36.5 g Zn 1 mol H2
Other possibilities: Still starting with 79.1 grams of HCl, how many molecules of H2 gas are produced? Or what mass of H2 gas is produced? How would these questions change your calculations? Zn HCl  ZnCl H2 79.1 g ? mc Or ? g Same conversion to moles, same mole to mole ratio, different last conversions… 79.1 g HCl 1 mol HCl 36.5 g Zn 1 mol H2 2 mol HCl 6.02 x 1023 mc H2 1 mol H2 = 6.5 x 1023 mc H2 = 2.17 g H2

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