S-C-8-1_The Mole Presentation

The Mole: A Shortcut for Chemists

Source: http://en.wikipedia.org/wiki/Mole_(animal)

The Mole

• The mole is a counting unit for chemists, the same way a baker uses a dozen.

• 1 dozen = 12 objects• 1 mole = 6.02 × 1023 objects • = 602,000,000,000,000,000,000,000 objects• That’s almost a trillion trillion!• 6.02 × 1023 is called Avogadro’s number.• “Mole” in writing; “mol” in calculations.

Representative Particles

• Moles can be used to count “representative particles:” atoms, molecules, ions, and formula units.

• The representative particle of an ionic compound is the formula unit.

• The representative particle of a covalent compound is the molecule.

• The representative particle of an element is the atom unless it is a diatomic element, in which case it is the molecule.

• 1 mole of any of the representative particles = 6.02 × 1023 of those particles.

• How many atoms in 1 mol of aluminum?• How many molecules are in 1 mol of water?• How many formula units are in 1 mol of salt?• How many atoms are in 0.5 mol of aluminum?• How many molecules are in 2 mol of water?• How many formula units are in 2.67 mol of salt?

Dimensional Analysis

Dimensional Analysis(Factor-Unit Method)

• A simple, powerful tool.

• Converts among measurements expressed in different units.

• Develops relationships between units and expresses this relationship as a factor of both units.

Dimensional Analysis(Factor-Unit Method)

• Example:– 500 grams is equivalent to how many pounds?

Dimensional Analysis(Factor-Unit Method)

• Example:– 500 grams is equivalent to how many pounds?

Step 1: Write the statement as a mathematical formula.

pounds______CFgrams500

Dimensional Analysis(Factor-Unit Method)

• Example:– 500 grams is equivalent to how many pounds?

Step 2: Write the relationship between the known and unknown units.

1 pound = 454 grams

Dimensional Analysis(Factor-Unit Method)

• Example:– 500 grams is equivalent to how many pounds?

Step 3: Write the relationship as two ratios.

grams 454pound 1

pound 1grams 454

Dimensional Analysis(Factor-Unit Method)

• Example:– 500 grams is equivalent to how many pounds?

Step 4: Select the relationship that has the unit sought on top.

grams 454pound 1

pound 1grams 454

pounds______CFgrams500

Dimensional Analysis(Factor-Unit Method)

• Example:– 500 grams is equivalent to how many pounds?

Step 4: Select the relationship that has the unit sought on top.

pounds______grams454

pound1grams500

Dimensional Analysis(Factor-Unit Method)

• Example:– 500 grams is equivalent to how many pounds?

Step 5: Perform the calculation.

pounds10.1grams454

pound1grams500

Dimensional Analysis(Factor-Unit Method)

• Example:– 500 grams is equivalent to how many pounds?

Step 6: Check the answer. Is it reasonable?

pounds10.1grams454

pound1grams500

Dimensional Analysis(Factor-Unit Method)

• Example:– A can of Coca-Cola contains 12 fluid ounces (fl oz).

What is the volume of the can in quarts? (Given: 1 qt = 32 fl oz)

Dimensional Analysis(Factor-Unit Method)

• Example: Solution– A can of Coca-Cola contains 12 fluid ounces (fl

oz). What is the volume of the can in quarts? (Given: 1 qt = 32 fl oz)

pounds______CFozfl12

Dimensional Analysis(Factor-Unit Method)

• Example: Solution– A can of Coca-Cola contains 12 fluid ounces (fl

oz). What is the volume of the can in quarts? (Given: 1 qt = 32 fl oz)

1 quart = 32 fl oz

pounds______CFozfl12

Dimensional Analysis(Factor-Unit Method)

• Example: Solution– A can of Coca-Cola contains 12 fluid ounces (fl

oz). What is the volume of the can in quarts? (Given: 1 qt = 32 fl oz)

quarts______oz fl 32

quart 1oz fl 21

quart 1 oz fl 23

oz fl 32quart 1

Dimensional Analysis(Factor-Unit Method)

• Example: Solution– A can of Coca-Cola contains 12 fluid ounces (fl

oz). What is the volume of the can in quarts? (Given: 1 qt = 32 fl oz)

quarts 38.0oz fl 32

quart 1oz fl 21

Atomic Masses

• What do the atomic masses on the periodic table represent?

• Carbon has an atomic mass of 12; this means a carbon atom weighs 12 atomic mass units (amu).

• The actual mass of an atom of carbon is only 2 x 10-23 grams.

• Practice: What is the atomic mass of one molecule of O2?

16.00 + 16.00 = 32.02 amu

Formula Weight

• Formula weight: The weight of a molecule or an ionic compound.

• Molecule: CO2

12.01 + 16.00 + 16.00 = 44.01 amuFor molecules, formula weight is also

called the “molecular weight.”• Ionic compound: NaCl 22.99 + 35.45 = 58.44 amu

Molar Mass

• Molar mass = the mass of one mole.• For atoms, molar mass is the same as atomic mass.• The molar mass of a compound allows you to convert

between the amount of the element (moles) and its mass (grams).

• If you want to convert from amount (moles) to mass (grams):

• If you want to convert from mass (grams) to amount (moles):

 # of moles ×  Formula weight = Mass (in grams)  1 mol  

mass (grams) × 1 mol = # of moles  formula weight    

Molar Mass Examples

• 1 mol N = 14.01 g N• Written as 14.01 g/mol• Calculate the following molar masses:

•Br•CaF2

•NO2

•NaCl

Molar Mass Examples: Solutions

Calculate the following molar masses:• Br

79.90• CaF2

40.08 + 19.00(2) = 78.08 g/mol• NO2

14.01 + 16.00(2) = 46.01 g/mol• NaCl

22.99 + 35.45 = 58.44 g/mol

Conversion Factors: Mole-Mass Problems

• The red portions in the examples above are the conversion factors.

• Conversion factor: A ratio equal to one that expresses the same quality in two different ways.

• Another example: To find the number of eggs in 3 dozen:3 dozen x 12 eggs/dozen = # of eggs

Mole-Mass Problems1. What is the mass of 0.50 mol HCl?

2. What is the mass of 2 mol Zn?

3. How many moles are in 1.5 g of Cu?

4. How many moles are in 50 g H2SO4?

Mole-Mass Answers1. What is the mass of 0.50 mol HCl?

0.50 mol × 36.46 g/mol = 18.23 g HCl

2. What is the mass of 2 mol Zn?2 mol × 65.41 g/mol = 130.82 g

3. How many moles are in 1.5 g of Cu?1.5 g × 1 mol/63.55 g = 0.02 mol

4. How many moles are in 50 g H2SO4?50 g × 1 mol/98.09 g = 0.51 mol