Section 3.6—Counting Molecules

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 The only acceptable abbreviation for “mole” is “mol”…not “m”!!

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

5. mL molecules H2O 6.021023 355 1.19  1025 = _________ molecules H2O mL 18 1.19  1025 molecules 1 dozen 9.89  1023 = _________ dozen molecules 12 Why can’t we count atoms in “dozens”? Atoms and molecules are extremely small There are 6.02  1023 water molecules in 18mL of water This means a 12 ounce bottle of water (355 mL) would have 1.19  1025 molecules of water. That would be 9.89  1023 “dozen” water molecules. These huge numbers are impractical!

6. What does a “mole” count in? 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 This means a 12 ounce bottle of water would have 19.7 “moles” of water…a much easier-to-work-with number!

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

8. 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

9. Let’s Practice #1 Example: How many moles are equal to 2.8 × 1022 molecules

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

11. Molar Mass

12. 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

13. 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

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

15. 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

16. Example: Molar Mass Example: Find the molar mass for CaBr2

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

18. 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

19. 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 =

20. 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

21. 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

22. 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

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

24. 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

25. Using Molar Mass in Conversions

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

27. 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 22.53 1 mol H2O

28. Example: Grams to Molecules Example: How many molecules are in 25.5 g NaCl?

29. 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 Molecules Example: How many molecules are in 25.5 g NaCl? 1 moles NaCl molecules = 58.44 g 1 mol = 6.021023 molecules 25.5 g NaCl mol NaCl molecules NaCl 6.021023 1 58.44 g NaCl 1 mol NaCl = _________ molecules NaCl 2.63  1023

30. Let’s Practice #3 Example: How many moles are in 25.5 g NaCl?

31. Na 1  22.99 g/mole 22.99 g/mole =  + 35.45 g/mole Cl 1 35.45 g/mole = 58.44 g/mole Let’s Practice #3 Example: How many moles are in 25.5 g NaCl? 1 mole NaCl molecules = 58.44 g 25.5 g NaCl mole NaCl 1 = _______ mole NaCl 0.44 58.44 g NaCl

32. Let’s Practice #4 Example: How many grams is a sample of 2.75 × 1024 molecules of SrCl2?

33. Sr 1  87.62 g/mole 87.62 g/mole =  + 70.90 g/mole Cl 2 35.45 g/mole = 158.52 g/mole Let’s Practice #4 Example: How many grams is a sample of 2.75 × 1024 molecules of SrCl2? 1 moles SrCl2 molecules = 158.52 g 1 mol = 6.021023 molecules 2.75 × 1024 molecules SrCl2 mol SrCl2 g SrCl2 1 158.52 6.02 × 1023 molecules SrCl2 1 mol SrCl2 = _________ g SrCl2 7.24