 # Section 3.6—Counting Molecules

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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. 500,000,000,000,000,000,000 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 x 1023 atoms = 1 mole Use dimensional analysis to go from particles to moles: 9.56 atoms Fe x 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
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

How many moles are equal to 2.8 × 1022 molecules
Practice #2 Example: How many moles are equal to 2.8 × 1022 molecules

How many moles are equal to 2.8 × 1022 molecules
Practice #2 Example: How many moles are equal to 2.8 × 1022 molecules 1 mol = 6.021023 molecules 2.8 × 1022 molecules 1 mole = _______ moles 0.047 6.02  1023 molecules

How is a mole related to mass?
What is molar mass?

Molar 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 Element Mass 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.
Determine the molar mass of lead. 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 CaCl2
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 = and Cl = 35.5 3 Multiple the # of atoms  mass for each atom 1 x 40.1 = 40.1 for Ca x 35.5 = 71.0 for Cl 4 Find the sum of all the masses = g/mol

Find the molar mass for CaBr2
Example 1: Molar Mass Example: Find the molar mass for CaBr2

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

Find the molar mass for CaBr2
Example 1: Molar Mass 2 Find the molar mass of each atom on the periodic table Example: Find the molar mass for CaBr2 Ca 1 40.1 g/mole Br 2 79.9 g/mole

Find the molar mass for CaBr2
Example 1: Molar Mass 3 Multiple the # of atoms  molar mass for each atom Example: Find the molar mass for CaBr2 Ca 1 40.1 g/mole = 40.1 g/mole Br 2 79.9 g/mole = 159.8 g/mole

Find the molar mass for CaBr2
Example 1: Molar Mass 4 Find the sum of all the masses Example: Find the molar mass for CaBr2 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 CaBr2 molecules would have a mass of g

Find the molar mass for Sr(NO3)2
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(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 87.6 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(NO3)2 molecules would have a mass of g

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

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
What is molar mass?

Example: Moles to Grams
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 O 2 1 1.0 g/mole 16.0 g/mole = 2.0 g/mole + 18.0 g/mole Finally, use dimensional analysis to get the answer! 1.25 mol H2O 18.0 g H2O = _______ g H2O 22.5 1 mol H2O

Example: Grams to Moles
Determine the number of moles in 58.6 g of Li(OH)2 58.6 g Li(OH) = ___ mol Li x = 6.9 g/mol O 2 x = 32.0 g/mol H 2 x = 2.0 g/mol 40.9 g/mol g Li(OH)2 x 1 mole = mol 40.9 g

Practice Determine the number of moles in 28.4 g of AgNO3.
2. What is the mass of 3.6 mol ZnO? 3. Determine the number of moles in 15.7 g of CO2. 4. What is the mass of 89.0 g of N2?

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
How many molecules are in 25.5 g NaCl?

Example: Grams to Molecules
How many molecules are in 25.5 g NaCl? Na Cl 1 23.0 g/mole 35.5 g/mole = + 58.5 g/mole 1 mol = 6.021023 molecules 25.5 g NaCl 1 mol NaCl 58.5 g NaCl = _________ molecules NaCl

Example: Grams to Molecules
How many molecules are in 25.5 g NaCl? Na Cl 1 23.0 g/mole 35.5 g/mole = + 58.5 g/mole 1 mol = 6.021023 molecules molecules NaCl 0.436 mol NaCl x 6.021023 1 mol NaCl = _________ molecules NaCl 2.62  1023

Example: Molecules to Grams
How many grams is a sample of 2.75 × 1024 molecules of SrCl2?

How many grams is a sample of 2.75 × 1024 molecules of SrCl2?
Let’s Practice #4 Example: How many grams is a sample of 2.75 × 1024 molecules of SrCl2? Sr Cl 1 2 87.6 g/mole 35.5 g/mole = 71.0 g/mole + 158.6 g/mole 1 mol = 6.021023 molecules 2.75 × 1024 molecules SrCl2 1 mol SrCl2 6.02 × 1023 molecules SrCl2 = _____ g SrCl2

How many grams is a sample of 2.75 × 1024 molecules of SrCl2?
Let’s Practice #4 Example: How many grams is a sample of 2.75 × 1024 molecules of SrCl2? Sr Cl 1 2 87.6 g/mole 35.5 g/mole = 71.0 g/mole + 158.6 g/mole 1 mol = 6.021023 molecules 4.57 mol SrCl2 158.6 g SrCl2 1 mol SrCl2 = _________ g SrCl2 724