The Mole: A Shortcut for Chemists

1. The Mole: A Shortcut for Chemists Source: http://en.wikipedia.org/wiki/Mole_(animal) S-C-8-1_The Mole Presentation

2. 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.

3. 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 atomunless it is a diatomic element, in which case it is the molecule.

4. 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?

5. Dimensional Analysis

6. 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.

7. Dimensional Analysis(Factor-Unit Method) • Example: • 500 grams is equivalent to how many pounds?

8. Dimensional Analysis(Factor-Unit Method) • Example: • 500 grams is equivalent to how many pounds? Step 1: Write the statement as a mathematical formula.

9. 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

10. Dimensional Analysis(Factor-Unit Method) • Example: • 500 grams is equivalent to how many pounds? Step 3: Write the relationship as two ratios.

11. 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.

12. 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.

13. Dimensional Analysis(Factor-Unit Method) • Example: • 500 grams is equivalent to how many pounds? Step 5: Perform the calculation.

14. Dimensional Analysis(Factor-Unit Method) • Example: • 500 grams is equivalent to how many pounds? Step 6: Check the answer. Is it reasonable?

15. 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)

16. 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)

17. 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

18. 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)

19. 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)

20. 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

21. Formula Weight • Formula weight: The weight of a molecule or an ionic compound. • Molecule: CO2 12.01 + 16.00 + 16.00 = 44.01 amu For molecules, formula weight is also called the “molecular weight.” • Ionic compound: NaCl 22.99 + 35.45 = 58.44 amu

22. 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):

23. 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

24. 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

25. Conversion Factors: Mole-Mass Problems • The redportions 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

26. Mole-Mass Problems 1. 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?

27. Mole-Mass Answers 1. 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