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Understanding Mole and Molar Mass in Chemistry

Learn how a mole is used as a counting unit and how to calculate molar mass for atoms and molecules in chemistry. Practice conversions between moles, particles, and mass.

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Understanding Mole and Molar Mass in Chemistry

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  1. 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?

  2. 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.

  3. 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.

  4. 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 6.02  1023 molecules 1 mole of molecules

  5. 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

  6. Practice #1: Molecules & Moles Example: How many molecules of water are in 1.25 moles?

  7. 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

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

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

  10. How is a mole related to mass?

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

  12. 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

  13. Examples Mass Element 12.0 g 1 mole of carbon atoms 16.0 g 1 mole of oxygen atoms 1 mole of hydrogen atoms 1.0 g

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

  15. 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.

  16. Practice: Calculate the Molar Mass of CaCl2 1 Count the number of each type of atom 1 Ca and 2 Cl Find the mass of each atom on the periodic table Ca = 40.1 and Cl = 35.5 2 Multiple the # of atoms  mass for each atom 1 x 40.1 = 40.1 for Ca 2 x 35.5 = 71.0 for Cl 3 4 Find the sum of all the masses 40.1 + 71.0 = 111.1 g/mol

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

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

  19. 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

  20. 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 =  159.8 g/mole Br 2 79.9 g/mole =

  21. 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 =  + 159.8 g/mole Br 2 79.9 g/mole = 199.9 g/mole 1 mole of CaBr2 molecules would have a mass of 199.9 g

  22. 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

  23. 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 =  + 96.0 g/mole O 6 16.0 g/mole = 211.6 g/mole 1 mole of Sr(NO3)2 molecules would have a mass of 211.6 g

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

  25. 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 =  + 3.0 g/mole H 3 1.0 g/mole = 78.0 g/mole 1 mole of Al(OH)3 molecules would have a mass of 78.0 g

  26. Conversions Between Moles & Mass

  27. Example: Moles to Grams Example: How many grams are in 1.25 moles of water?

  28. H 2  1.0 g/mole 2.0 g/mole =  + 16.0 g/mole O 1 16.0 g/mole = 18.0 g/mole Example: Moles to Grams First determine the given quantity and the unknown one! Nextdetermine the molar mass for the compound. Example: How many grams are in 1.25 moles of water? Finally, use dimensional analysis to get the answer! 1.25 mol H2O g H2O 18.0 = _______ g H2O 22.5 1 mol H2O

  29. Example: Grams to Moles • Determine the number of moles in 58.6 g of Li(OH)2 • 58.6 g Li(OH)2 = ___ mol • Li 1 x 6.9 = 6.9 g/mol • O 2 x 16.0 = 32.0 g/mol • H 2 x 1.0 = 2.0 g/mol • 40.9 g/mol • 3. 58.6 g Li(OH)2 x 1 mole = 1.43 mol • 40.9 g

  30. 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?

  31. 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.

  32. 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.

  33. Example: Grams to Molecules Example: How many molecules are in 25.5 g NaCl?

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

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

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

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

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

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