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Lect 3 Stoichiometry Calculations-2010

# Lect 3 Stoichiometry Calculations-2010 - Calculations with...

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Calculations with Chemical Formulas and Equations Stoichiometry Mass Relationships in Chemical Compounds and Chemical Reactions

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Composition Stoichiometry How do you relate the “microscopic” masses of atoms and molecules to their “macroscopic” gram quantities? What is a mole quantity? How do you relate mole quantities to gram quantities and grams to moles? How do you convert chemical formulas to mass % compositions? How do you convert mass % compositions to chemical formulas ?
Reaction Stoichiometry Calculation of quantitative (measurable) relationships of the reactants and products in a balanced chemical reaction How do you calculate the mass of reactants necessary for complete reaction? How do you calculate the mass of products obtained from given amounts of reactants? How do you calculate the theoretical yield of products of a reaction? How do you treat reaction stoichiometry for reactants in solution?

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Atomic Weight Weighted average of the masses of the naturally occurring isotopes in an element relative to the mass of 12 C = 12.00 amu Atomic mass unit (amu) 12

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Molecular Weight and Formula Weight The molecular weight of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. H = 1.0 amu; O = 16.0 amu 2
Mass and Moles of a Substance A mole is the quantity of a given substance that contains as many molecules (or formula units) as the number of

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Mass and Moles of a Substance The molar mass of a substance is the mass of one mole of a substance. molar mass , in grams per mole, is numerically equal to the formula weight in atomic mass units. One mole of any element weighs its atomic mass in grams.
1 mole AW = MW (FW) in g No. of atoms or molecules 12 C 12.00 g 6.02 x 10 23 23

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i.e. there are 6.02 x 10 atoms or molecules mole of the substance in grams. Since atomic mass = g/mol 23 Avagadro number (N) = 6.02 x 10 23 /mol
Calculation of Mass Moles of a Substance Moles of A = Mass of A AW or MW of A How many moles of aluminum (Al) are there in 100 g of aluminum (atomic mass = 27.0 g/mol) ? Mole of Al = 100 g of Al x 1 mole Al = 3.71 mol of Al 27.0 g Al What is its mass in grams of 5.75 moles of magnesium (atomic mass. = 24.3 g/mol)? 5.75 mole of Mg x 24.3 g Mg = 140 g Mg 1 mole Mg

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Mass and Moles of a Substance How many moles of H 2 O (18.0 g/mol) in 100.0 grams of H 2 O? 2 2 2 2 What is its mass in grams of 3.25 moles of glucose, C 6 H 12 O 6 (MW = 180.0 g/mol)? 6 12 6 6 12 6
Mass and moles to number of molecules or atoms How many molecules of HCl are there in a 3.46 g sample of HCl? (Atomic Mass : H = 1.00, Cl = 35.5) 1 mole of HCl = 6.02 x 10 23 molecules of HCl 23 g → mole → molecules

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How many atoms of gold (Au) are there in 1.55 g of Au? (Atomic Mass: Au = 197 g/mol) 23 g → mole → atoms
mg → g → mole → molecules → atoms How many carbon atoms are there in 4.00 μ g of propane, C 3 H 8 ? (Atomic Mass: C = 12.0, H = 1.00) Avagadro number (N) = 6.02 x 10 23 /mol 3 8 3 8 3 8 23 3 8

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Avagadro Number = 6.02 x 10 -9
The mass percentage of an element A in a compound is the mass of A per hundred grams of the compound Mass% of A = mass of A in compound x 100 mass of compound Mass Percentages from Chemical Formulas

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