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Understanding Atomic Weights and Mole Concept

Learn about atomic weights, elements, and moles in chemistry. Discover how to calculate formula weight and molecular weight. Understand Avogadro's number and the significance of molar mass.

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Understanding Atomic Weights and Mole Concept

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  1. Atomic Weights • The mass of an individual atom, ion, or molecule is very small. • Scientists use the atomic mass unit (amu) to express the mass of atoms or molecules. • 1 amu = 1.66054 x 10-24 g

  2. Atomic number Atomic weight (atomic mass) 5 10.81 B Elementsymbol electronegativity en = 2.04 boron Element name Atomic Weight • Atomic weight (atomic mass): • the average mass of one atom of an element expressed in amu • found on the periodic table • The atomic weight of an element is reported as the weighted average of the naturally occurring isotopes.

  3. Atomic Weights • The atomic weight for silicon is found by taking a weighted average of the masses of the three naturally occurring isotopes of silicon: 28Si : 92.2297% of 27.9769 amu = 25.803 amu 29Si : 4.6832% of 28.9765 amu = 1.3570 amu 30Si: 3.0872% of 29.97377 amu = 0.92535 amu AW of Si = 28.085 amu

  4. Formula Weight • The mass of a compound can be expressed using either its formula weight (all compounds) or its molecular weight (molecular compounds only). • Formula Weight: • the sum of the atomic weights of all of the atoms in any chemical formula • Molecular Weight: • the sum of the atomic weights of all of the atoms in a molecular formula

  5. Formula Weight and Molecular Weight • Formula weight and molecular weight are found by the exact same process: • The molecular weight of CO2 is calculated as follows: FW = 1 (AW of C) + 2 (AW of O) = 1 (12.01 amu) + 2 (16.00 amu) = 44.01 amu

  6. Formula Weight and Molecular Weight • The formula weight of CaCl2 is calculated as follows: • FW = 1(AW of Ca) + 2(AW of Cl) Ca: 1 (40.08 amu) = 40.08 amu Cl: 2 (35.45 amu) = 70.90 amu 110.98 amu

  7. Formula Weight and Molecular Weight • How many significant figures should I use for the atomic weight??? • Use enough significant figures in the AW to avoid limiting the number of significant figures in any other calculation. • I want you to always use at least 2 decimal places (rounded appropriately) in the AW’s that you use in a calculation.

  8. Molecular Weight Example: Calculate the MW of phosphoric acid Phosphoric acid = ????

  9. Mole • Atoms and molecules are very small. • 1 cm length • 20 to 50 million atoms • Tiny speck of carbon • 60 million billion atoms (6 x 1016) • 1 cm3 box • billions of trillions of atoms

  10. Mole • Virtually impossible to manually count the # of atoms in a reasonable size sample • Tiny speck of carbon • 60 million billion atoms (6 x 1016) • Counting 1 atom per second, 24 hours per day, 365 days per year it would take: • ~ 2 billion years

  11. H H + O O H H Mole • Knowing the number of atoms or molecules in a sample is important. • Chemical reactions occur on an atomic or molecular level. 2 H2 + O2 2 H2O

  12. Mole • Chemists usually measure out chemicals in grams not as individual atoms or molecules. • Need a way to • count the number of atoms or molecules in a sample and • relate the number of atoms or molecules to the mass of a sample

  13. Mole • Mole: • the amount of a material that contains the same # of units as there are atoms in exactly 12 g of C-12 • the amount of material that contains 6.022 x 1023 atoms (or molecules or ions or any other chemical species)

  14. Mole 1 mole 12C atoms = 6.022 x 1023 12C atoms 1 mole H2O molecules = 6.022 x 1023 H2O molecules 1 mole Na+ ions = 6.022 x 1023 Na+ ions

  15. Mole • Simply a way to count, express, and compare the number of atoms or molecules present • pair of gloves (2) • dozen eggs (12) • case of pop (24) • gross (144) • Mole (contains 6.022 X 1023 units)

  16. Avogadro’s Number • Avogadro’sNumber (N):the number of atoms, ions, or molecules per mole • a constant • N = 6.022 x 1023atoms, ions or molecules mole

  17. Avogadro’s Number • How big is Avogadro’s Number? If you have a mole of pennies (i.e. Avogadro’s number) and divide them equally among the 6 billion people on Earth, how many dollars would each person get?

  18. = $1 x 1012 person x $1 100 pennies Avogadro’s Number 6.022 X1023 pennies 6 x 109 people Each person would get ~ $ 1 trillion!!!

  19. Comparing moles of different materials • 1 mole Cu = 6.022 x 1023 atoms = 63.55 g Cu 22 copper (pre-1983) pennies

  20. Comparing moles of different materials • 1 mole He = 6.022 x 1023 atoms = 4.003 g He ~7 medium-sized He-filled ballons

  21. Comparing moles of different materials • 1 mole H2O = 6.022 x 1023 molecules of H2O = 18.016 g H2O • ~18 mL of water

  22. Mole • How do we relate the number of atoms (or molecules) in a mole to mass??? • 1 mole = 6.022 x 1023 atoms • The mass of 6.022 x 1023 atoms is a unique number for each element. • Atoms of each element have different masses • 1 mole of each element has a unique mass (molar mass)

  23. Molar Mass • Molar mass • the mass in grams of 1 mole of substance • numerically equal to its formula weight (for ionic compounds), molecular weight (for molecules), or atomic weight (for atoms)

  24. 1 NO3- ion has a mass of 62.0 amu 1 mol NO3- ions has a mass of 62.0 g Molar Mass • 1 H2O molecule has a mass of 18.0 amu 1 mol H2O has a mass of 18.0 g

  25. Molar Mass • Molar mass can be used as a conversion factor: • Relates the # grams and # moles • 1 mole of N2 has a mass of 28.0 g 28.0 g N2 1 mole N2 1 mole N2 28.0 g N2 OR

  26. Calculating the number of moles in a sample Example: A medium-sized balloon contains 0.55 g of He gas. How many moles of He are in the balloon?

  27. Calculating the # of moles Example:How many moles are present in 25.0 g of acetic acid ?

  28. Calculating mass from moles Example:A 1-carat diamond, which is pure carbon, contains 0.0167 mol of carbon. What is the mass, in grams, of the diamond?

  29. Calculating mass from moles Example:What is the mass in grams of 0.500 moles of carbon dioxide?

  30. Avogadro’s Number • Avogadro’s number can also be used as a conversion factor: 1 mole = 6.022 x 1023 molecules (or atoms, ions, formula units) 6.022 x 1023 atoms 1 mole 1 mole 6.022 x 1023 atoms OR

  31. Calculating # of molecules from # of moles Example:How many water molecules are in 2.5 moles of water?

  32. Calculating # moles from # of molecules Example:How many moles are in 3.2 x 1022 molecules of water?

  33. Use molar mass grams moles molecules Calculating Mass from # of Molecules or Vice Versa Use Avogadro’s number

  34. Calculating mass from molecules Example:What is the mass in grams of 3.0 x 1024 molecules of nitric acid?

  35. Calculating mass from molecules

  36. Calculating # of molecules from mass Example:How many molecules are in a 5.0 g sample of hydrogen?

  37. Calculating # of molecules from mass

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