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Chemical Measurements Unit 1: Stoichiometry Chapter 10 – The Mole

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Monday, 2/2 Learning Target: Distinguish between atomic mass, formula mass and molar mass. Learning Outcome: Complete “Formula Mass and Molar Mass” Worksheet.

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Measurements in Chemistry We use measurements all of the time! Atomic Mass is a measurement that we have already used in class. o units for atomic mass = atomic mass unit ( amu) o Used to express masses of atoms on a relative scale. We compare everything to carbon-12.

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Particlelocation Mass (g) Mass (amu) Proton Inside nucleus 1.673X10 -24 1.0073= 1 Neutron Inside nucleus 1.675X10 -24 1.0087= 1 Electron Outside nucleus 7.109X10 -28 0.0006= 0

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Atomic Mass & Formula Mass Atomic Mass o The weighted average of the masses of the existing isotopes of an element o Ex. Carbon C 12.01 amu Formula Mass o The sum of the atomic masses of all atoms in a compound o Ex. Carbon Dioxide CO 2 44.01 amu

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Calculating Formula Mass Methylene chloride (CH 2 Cl 2 ) is used as a solvent in paint strippers. What is the formula mass of methylene chloride? o C: 1 atom x 12.01 amu o H: 2 atoms x 1.01 amu o Cl: 2 atoms x 35.45 amu Formula Mass = 12.01(1) + 1.01(2) + 35.45(2) = 83.93 amu

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Calculating Formula Mass What is the formula mass for SO 2 ? o S: 1 atom x 32.07 amu o O: 2 atoms x 16.00 amu Formula Mass = 32.07 amu + 16.00 amu (2)= 64.07 amu

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Calculating Formula Mass What is the formula mass for H 2 O 2 ? o H: 2 atoms x 1.01 amu o O: 2 atoms x 16.00 amu Formula Mass = 2.02 amu + 32.00 amu = 34.02 amu

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Moving from AMU to grams The use of atomic mass units (amu) are impractical in the chemistry lab where the preferred unit of measurement is grams. Scientists needed to establish a relationship between # of atoms and masses of atoms.

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Moving from AMU to grams We are missing something! We don’t know how many atoms are necessary to make up a mass in grams that is equal to an element’s atomic mass We need some sort of conversion factor

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The Mole Definition o The number of atoms of that element equal to the number of atoms in exactly 12.0 grams of carbon- 12. o Abbreviated mol (without the ‘e’) The number of atoms in one mole of atoms is always the same! Avogadro’s Number (N) o 6.02 x 10 23 atoms o 602,000,000,000,000,000,000,000

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Same Number, Different Mass Element # atoms/mol mass of 1 mole Carbon 6.02x10 23 atoms 12.01 g C Copper 6.02x10 23 atoms 63.55 g Cu Tin 6.02x10 23 atoms 118.71 g Sn Why do the amounts of each look different?

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Molecules and Moles The number of molecules in 1mole of a molecular compound is 6.02 x 10 23 (same as with atoms in a mole) 1 mol of water (H 2 O) contains 1 mol of water molecules but 2 mol of hydrogen atoms and 1 mol oxygen. How many moles of Ca 2+ and F - are in 1 mole of calcium fluoride (CaF 2 )? o 1 mole of Ca 2+ ions and 2 moles of F - ions

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Molar Mass Definition o The mass in grams of 1 mole of a substance We calculate molar mass the same way as formula mass. o Atomic/Formula Molar Mass o Ex. Calcium 40.08 amu = 40.08 g/mol

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Calculating Molar Mass Methylene chloride (CH 2 Cl 2 ) is used as a solvent in paint strippers. What is the molar mass of methylene chloride? o C: 1 atom x 12.01 amu o H: 2 atoms x 1.01 amu o Cl: 2 atoms x 35.45 amu Formula Mass = 12.01(1) + 1.01(2) + 35.45(2) = 84.95 amu Molar Mass = 84.95 g/mol

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Same Number, Different Mass Practice What is the formula and molar mass for propane, C 3 H 8 ? –C: 3 atoms x 12.01 amu = 36.03 amu –H: 8 atoms x 1.01 amu = 8.08 amu Formula Mass = 36.03 amu + 8.08 amu = 44.11 amu Molar Mass = 44.11 g/mol

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Same Number, Different Mass Practice What is the formula and molar mass for glucose, C 6 H 12 O 6 ? –C: 6 atoms x 12.01 amu = 72.06 amu –H: 12 atoms x 1.01 amu = 12.12 amu –O: 6 atoms x 16.00 amu = 96.00 amu Formula Mass = 72.06 amu + 12.12amu + 96 amu = 180.18 amu Molar Mass = 180.18 g/mol

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