# Chemical Measurements Unit 1: Stoichiometry Chapter 10 – The Mole.

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Chemical Measurements Unit 1: Stoichiometry Chapter 10 – The Mole

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

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.

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

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

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

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

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

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.

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

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

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?

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

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

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

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

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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