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Chemical Measurements Unit 4: Stoichiometry Chapter 10 – The Mole Overview • Measurements are essential. • Measuring the number of atoms involved in a chemical reaction is impractical. • The mole establishes a relationship between the number of particles involved in a reaction and the mass of that reactant or product. • Mole relationships are used to determine % composition, empirical and molecular formulae. Measurements • We use measurements all of the time! • Atomic Mass is a measurement that we have already used in class. o o o units for atomic mass = amu atomic mass unit Used to express masses of atoms on a relative scale. We compare everything to carbon-12. Particle location Mass (g) Mass (amu) Inside nucleus 1.673X10-24 1.0073=1 Inside Neutron nucleus 1.675X10-24 1.0087=1 Outside Electron nucleus 7.109X10-28 0.0006=0 Proton Atomic Mass & Formula Mass • Atomic Mass The weighted average of the masses of the existing isotopes of an element o Ex. Carbon C 12 amu o • Formula Mass The sum of the atomic masses of all atoms in a compound o Ex. Carbon Dioxide CO2 44 amu o Calculating Formula Mass • Methylene chloride (CH2Cl2) is used as a solvent in paint strippers. What is the formula mass of methylene chloride? o o o C: 1 atom x 12.01 amu H: 2 atoms x 1.01 amu Cl: 2 atoms x 35.45 amu • Formula Mass = 12.01(1) + 1.01(2) + 35.45(2) = 83.92 amu Calculating Formula Mass • What is the formula mass for SO2? o o S: 1 atom x 32.07 amu 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 H2O2? o o H: 2 atoms x 1.01 amu 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 The number of atoms of that element equal to the number of atoms in exactly 12.0 grams of carbon12. o Abbreviated mol (without the ‘e’) o • The number of atoms in one mole of atoms is always the same! o 6.02 x 1023 atoms Same Number, Different Mass Element # atoms/mol mass of 1 mole Copper 6.02x1023 atoms Cu 63.55 g Mercury 6.02x1023 atoms Hg 200.59 g Sulfur 6.02x1023 atoms S 32..07 g Iron 6.02x1023 atoms Fe 55.85 g Molecules and Moles • The number of molecules in any molecular compound is 6.02 x 1023 • 1 mole of water (H2O) contains 1 mole of water molecules but 3 moles of atoms. • How many moles of Ca2+ and F- are in 1 mole of calcium fluoride (CaF2)? o 1 mole of Ca2+ ions and 2 moles of F- ions Avogadro’s Number • 6.02 x 1023 is the quantity of items in a mole is known as Avogadro’s Number (abbreviated N) in honor of Amadeo Avogadro. 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 amu = 40.08 g Calculating Molar Mass • Methylene chloride (CH2Cl2) is used as a solvent in paint strippers. What is the molar mass of methylene chloride? o o o C: 1 atom x 12.01 amu H: 2 atoms x 1 amu Cl: 2 atoms x 35 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, C3H8? – 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, C6H12O6? – C: 6 atoms x 12 amu = 72 amu – H: 12 atoms x 1 amu = 12 amu – O: 6 atoms x 16 amu = 96 amu Formula Mass = 72 amu + 12 amu + 96 amu = 180 amu Molar Mass = 180 g/mol