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Understanding The Mole in Chemistry

Explore the concept of the mole in chemistry, its importance in counting atoms and molecules, and how to utilize dimensional analysis and molar mass calculations. Learn to convert measurements and solve mole-mass problems effectively.

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Understanding The Mole in Chemistry

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  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

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