Section 3 6Counting Molecules Objectives Define a mole

Section 3. 6—Counting Molecules Objectives • Define a mole and Avogadro’s number • Define and calculate molar mass • Define molecular and atomic mass as mass for 1 mole of particles • Perform calculations between numbers of moles, particles, and masses So the number of molecules affects pressure of an airbag…how do we “count” molecules?

What is a mole? Mole – SI unit for counting The only acceptable abbreviation for “mole” is “mol”…not “m”!! You have had experience with counting units before: Dozen = counting unit that means “ 12” A dozen eggs is 12 eggs. Pair = counting unit that means “ 2” A pair of mittens is 2 mittens.

The Mole Since atoms and molecules are extremely small, even if we have a small sample, the number of molecules would be overwhelming. For example, one drop of water contains about 5 x 1020 molecules. 500, 000, 000 molecules!! These huge numbers are impractical! Using the “mole” counting unit, it becomes more manageable. 1 drop of water is about 8 x 10 -4 (0. 0008) moles.

The Mole 1 mole of anything = 6. 02 1023 of those things (This number is called Avogadro’s number) “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

Converting Between Moles & Molecules Remember: 1 mole = 6. 02 x 1023 particles Use dimensional analysis to go from moles to particles: 3. 24 mol Fe x 6. 02 x 1023 atoms = 1 mole Use dimensional analysis to go from particles to moles: 9. 56 atoms Fe x 1 mole = 6. 02 x 1023 atoms

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

Practice #1: 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 = _______

Practice #2 Example: How many moles are equal to 2. 8 × 1022 molecules

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

How is a mole related to mass?

Molar Mass – The mass for one mole of atoms or molecules. Unit for molar mass: g/mole or g/mol

For Elements Mass of 1 mole = the atomic mass of the element in grams Recall: Atomic mass is found on the periodic table. In THIS CLASS, we will round the atomic mass to the nearest 0. 1 g

Examples Mass Element 1 mole of carbon atoms 12. 0 g 1 mole of oxygen atoms 16. 0 g 1 mole of hydrogen atoms 1. 0 g

Practice 1. Determine the molar mass of helium. 2. Determine the molar mass of lead. 3. What is the mass of 1 mole of chlorine atoms?

Molar Mass for Compounds Mass of 1 mole = *SUM of the atomic masses of all elements in the compound, in grams *The sum of the atomic masses of all the elements in a compound is also called the formula mass or the gram formula mass.

Practice: Calculate the Molar Mass of Ca. Cl 2 1 Count the number of each type of atom 1 Ca and 2 Cl 2 Find the mass of each atom on the periodic table Ca = 40. 1 and Cl = 35. 5 3 Multiple the # of atoms mass for each atom 1 x 40. 1 = 40. 1 for Ca 4 2 x 35. 5 = 71. 0 for Cl Find the sum of all the masses 40. 1 + 71. 0 = 111. 1 g/mol

Example 1: Molar Mass Example: Find the molar mass for Ca. Br 2

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

Example 1: 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. 1 g/mole Br 2 79. 9 g/mole

Example 1: Molar Mass 3 Example: Find the molar mass for Ca. Br 2 Multiple the # of atoms molar mass for each atom Ca 1 40. 1 g/mole = 40. 1 g/mole Br 2 79. 9 g/mole = 159. 8 g/mole

Example 1: Molar Mass 4 Example: Find the molar mass for Ca. Br 2 Find the sum of all the masses Ca 1 40. 1 g/mole = 40. 1 g/mole Br 2 79. 9 g/mole = + 159. 8 g/mole 199. 9 g/mole 1 mole of Ca. Br 2 molecules would have a mass of 199. 9 g

Example 2: Molar Mass 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. 6 g/mole = 87. 6 g/mole N 2 14. 0 g/mole = 28. 0 g/mole O 6 16. 0 g/mole = + 96. 0 g/mole 211. 6 g/mole 1 mole of Sr(NO 3)2 molecules would have a mass of 211. 6 g

Let’s Practice #1 Example: Find the molar mass for Al(OH)3

Let’s Practice #1 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 27. 0 g/mole = 27. 0 g/mole O 3 16. 0 g/mole = 48. 0 g/mole H 3 1. 0 g/mole = + 3. 0 g/mole 78. 0 g/mole 1 mole of Al(OH)3 molecules would have a mass of 78. 0 g

Conversions Between Moles & Mass

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

Example: Moles to Grams First determine the given quantity and the unknown one! Next determine the molar mass for the compound. Example: How many grams are in 1. 25 moles of water? H 2 1. 0 g/mole O 1 16. 0 g/mole 2. 0 g/mole = = + 16. 0 g/mole 18. 0 g/mole Finally, use dimensional analysis to get the answer! 1. 25 mol H 2 O 18. 0 g H 2 O 1 mol H 2 O 22. 5 = _______ g H 2 O

Example: Grams to Moles Determine the number of moles in 58. 6 g of Li(OH)2 1. 58. 6 g Li(OH)2 2. Li 1 x 6. 9 O 2 x 16. 0 H 2 x 1. 0 = ___ mol = 6. 9 g/mol = 32. 0 g/mol = 2. 0 g/mol 40. 9 g/mol 3. 58. 6 g Li(OH)2 x 1 mole = 1. 43 mol 40. 9 g

Practice 1. Determine the number of moles in 28. 4 g of Ag. NO 3. 2. What is the mass of 3. 6 mol Zn. O? 3. Determine the number of moles in 15. 7 g of CO 2. 4. What is the mass of 89. 0 g of N 2?

Converting From Particles to Mass We have learned how to convert from the number of particles to moles and vice versa. We use 1 mol = 6. 02 x 1023 as the conversion factor. We have learned how to convert from number of moles to a mass and vice versa. We use 1 mol = molar mass (g) as the conversion factor.

Converting From Particles to Mass Number of Particles Mass 1 mol = 6. 02 x 1023 particles 1 mol = molar mass (g/mol) Mole We can convert from number of particles to mass in 2 separate steps.

Example: Grams to Molecules Example: How many molecules are in 25. 5 g Na. Cl?

Example: Grams to Molecules Example: How many molecules are in 25. 5 g Na. Cl? 25. 5 g Na. Cl Na 1 23. 0 g/mole Cl 1 35. 5 g/mole = 23. 0 g/mole = + 35. 5 g/mole 58. 5 g/mole 1 mol = 6. 02 1023 molecules 1 58. 5 mol Na. Cl g Na. Cl = _____ molecules Na. Cl

Example: Grams to Molecules Example: How many molecules are in 25. 5 g Na. Cl? Na 1 23. 0 g/mole Cl 1 35. 5 g/mole = 23. 0 g/mole = + 35. 5 g/mole 58. 5 g/mole 1 mol = 6. 02 1023 molecules 0. 436 mol Na. Cl x 6. 02 1023 molecules Na. Cl 1 mol Na. Cl 2. 62 1023 molecules Na. Cl = _____

Example: Molecules to Grams Example: How many grams is a sample of 2. 75 × 1024 molecules of Sr. Cl 2?

Let’s Practice #4 Example: How many grams is a sample of 2. 75 × 1024 molecules of Sr. Cl 2? 2. 75 × 1024 molecules Sr. Cl 2 Sr 1 87. 6 g/mole Cl 2 35. 5 g/mole = 87. 6 g/mole = + 71. 0 g/mole 158. 6 g/mole 1 mol = 6. 02 1023 molecules 1 mol Sr. Cl 2 6. 02 × 1023 molecules Sr. Cl 2 = _____ g Sr. Cl 2

Let’s Practice #4 Example: How many grams is a sample of 2. 75 × 1024 molecules of Sr. Cl 2? Sr 1 87. 6 g/mole Cl 2 35. 5 g/mole = 87. 6 g/mole = + 71. 0 g/mole 158. 6 g/mole 1 mol = 6. 02 1023 molecules 4. 57 mol Sr. Cl 2 158. 6 1 g Sr. Cl 2 mol Sr. Cl 2 724 = _____ g Sr. Cl 2