Ch400Ch3LN2 - Chem 400 Chapter 3 Lecture Notes Part 2...

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Chem 400 Chapter 3 Lecture Notes Part 2 Molecular or Formula Masses, Avogadro&s Number, Moles, and Molar Masses: ¥ You learned that the number underneath an Elemental Symbol on the Periodic Table is its atomic mass (the mass of 1 &average± atom in amu). ¥ What about a formula like (NH 4 ) 3 PO 4 or C 6 H 12 O 6 ? How do we calculate the formula mass? ¥ It²s just the sum of all the atomic masses in the formula. ¥ OK that²s easy, but is a formula mass very useful in the lab? ¥ Here²s the balanced equation for the rxn of hydrogen gas with oxygen gas: 2H 2 (g) + O 2 (g) & 2H 2 O(g) ¥ Reading this, we say: 2 molecules of hydrogen gas react with 1 molecule of oxygen gas to produce 2 molecules of water vapor. ¥ Can we weigh out individual molecules or ions on a lab balance? ¥ No, chemists weigh out compounds and elements in grams, which contain 10 23 molecules or atoms or ions. ¥ So chemists needed a way to express large amounts of molecules easily without scientific notation. ¥ Well since chemists already had an amu system of atomic or formula masses, they thought that it would be convenient to define a counting unit so that they could use the same mass for measuring grams as well. ¥ This counting unit (just like a dozen is a counting unit equal to 12) is called the mole (abbreviated mol) where:
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Ch400Ch3LN2 - Chem 400 Chapter 3 Lecture Notes Part 2...

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