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3.3 Atomic and Molecular Weights

3.3 Atomic and Molecular Weights. Demo: large bag of Styrofoam peanuts, bottle of sand, 1 L beaker, 18 mL of water How can we count very large numbers of particles? Demo: weigh pennies of various ages. Masses of Coins. Masses of Coins.

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3.3 Atomic and Molecular Weights

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  1. 3.3 Atomic and Molecular Weights • Demo: large bag of Styrofoam peanuts, bottle of sand, 1 L beaker, 18 mL of water • How can we count very large numbers of particles? • Demo: weigh pennies of various ages

  2. Masses of Coins

  3. Masses of Coins If we have a large number of particles of two close but different masses, how do we describe the mass of these particles?

  4. Atomic and Molecular Weights • How do we accommodate the masses of isotopes of an element?

  5. Atomic and Molecular Weights • Why do we use a C-12 standard for the mass of atoms of elements? • How can we determine these masses?

  6. Mass Spectrometry of Cl Atoms • How do we get average atomic weights? • When would the average atomic weight not be a useful number?

  7. Atomic and Molecular Weights • atomic weight: average mass of 1 atom of an element, expressed in amu • formula weight: sum of the atomic weights of each atom in a chemical formula • What is the formula weight of CaCl2? • molecular weight: same as formula weight when the chemical formula is a molecular formula • What is the molecular weight of H2CO3?

  8. Percentage Composition from Formulas • Percent composition is the atomic weight for each element divided by the formula weight of the compound multiplied by 100:

  9. 3.4 The Mole • Demo: 1 mole of different substances • What do these substances all have in common?

  10. Hg Fe2O3 CuSO4.5H2O NaCl Sn S8 Cu H2O Fe Avogadro’s number: NA = 6.022 x 1023 particles/mole

  11. Molar Mass and Moles • Describe the difference between molar mass, molecular weight, and atomic weight. • How do we get these quantities? • Calculations: mass  moles  number of particles • How do we carry out these conversions? • mass  moles: Use molar mass • moles  number of particles: Use NA

  12. Mole Calculations • Write on the blackboard and calculate the number of CaCO3 units in the writing. • How do we determine the mass of the writing? • How much Ca, C, and O is in the writing?

  13. Weigh the chalk before and after writing • Mass of writing = 5.473 g - 5.448 g = 0.025 g

  14. How much Ca, C, O in the writing? • 0.025 g x 1 mol/100.08 g = 0.00025 mol CaCO3 • 0.00025 mol CaCO3 contains 0.00025 mol Ca, 0.00025 mol C, 0.00075 mol O • 0.00025 mol Ca x 40.08 g/mol = 0.0100 g Ca • 0.00025 mol C x 12.01 g/mol = 0.00300 g C • 0.00075 mol O x 16.00 g/mol = 0.0120 g O • sum = 0.0100 + 0.00300 + 0.0120 = 0.0250 g

  15. 3.5 Empirical Formulas from Analyses

  16. Empirical Formulas • Calculate the empirical formulas: • 50% O, 50% S • 60% O, 40% S • Assume 100 g of the material: • 50 g O x 1 mol/16.0 g = 3.125 mol O • 50 g S x 1mol/32.066 g = 1.559 mol S • mol O/mol S = 3.125/1.559 = 2.004 or 2 • SO2 • 60 g O, 40 g S  3.75 mol O, 1.25 mol S • SO3

  17. Molecular formula from empirical formula • How to calculate a molecular formula if a molar mass is known? • Empirical formula = CH2O, MM = 90 g/mol • What is the molecular formula? • Empirical formula mass = 12 + 2 + 16 = 30 • The molar mass is 3 x as great, so the molecular formula is three empirical formulas: (CH2O)3 or C3H6O3

  18. Group Quiz Format • Place TA name and section letters (BA, or BB, or … , or BH) in upper left corner of paper. • Place name of group members participating in the quiz in the upper right corner. Please use correct spelling and write or print legibly. • Show work for partial credit.

  19. Group Quiz 4 • A compound contains 71.0% potassium and 29.0 % oxygen. The molar mass of the compound is about 110 g/mol. • What is the empirical formula of the compound? • What is the molecular formula of the compound?

  20. Answers to group quiz • Assume a sample of 100 g, which then contains 71.0 g K and 29.0 g O. • 71.0 g K x 1 mol/39.1 g = 1.82 mol K • 29.0 g O x 1 mol/16.0 g = 1.81 mol O • ratio = 1.82 mol K/1.81 mol O = 1.00 • empirical formula = KO • formula weight = 39.1 + 16.0 = 55.1 • molar mass/formula weight = 110/55.1 = 2 • formula = (KO)2 = K2O2

  21. 3.6 Quantitative Information from Balanced Equations • Mass-Mole Conversions • 10 g CaCO3 How many moles?MM = 100 g/mol10 g x 1 mol/100 g = 0.10 mol • How many moles in 20 g? • How many moles in 25 g of NaOH?MM = 40 g/mol

  22. Calculations • Mole-Mole Conversions • CaCl2 + Na2CO3 CaCO3 + 2NaCl0.105 mol xs ? ?How many moles of each product?

  23. Mass - Mass Conversions • Mass-Mass Conversions

  24. Calculations • Mass Conversions in a Single Reaction • CaCl2 + Na2CO3 CaCO3 + 2NaCl 5.45 g ? ?111 g/mol 100.1 g/mol 58.4 g/mol • Mass Conversions in Sequences of ReactionsFollow the same sequence of conversions, using the amounts of products from the first reaction as the amount of reactant in the second reaction.

  25. 3.7 Limiting Reactants • Limiting Reactant Demo: Mg or Zn in HCl • Analogy: making cheese sandwiches 05m05vd1

  26. Limiting Reactant

  27. Limiting Reactant

  28. Limiting Reactant 2H2 + O2 2H2O What is the limiting reactant?

  29. Limiting Reactant 2H2 + O2 2H2O What is the limiting reactant?

  30. Limiting Reactant 2H2 + O2 2H2O What is the limiting reactant?

  31. Limiting Reactant Problems

  32. Calculations • CaCl2 + Na2CO3 CaCO3 + 2NaCl 0.105 mol 0.085 mol ? ?How many moles of each product? • CaCl2 + Na2CO3 CaCO3 + 2NaCl 5.45 g 4.55 g ? ?111 g/mol 106 g/mol 100.1 g/mol 58.4 g/molHow much CaCO3 (in g) is formed?

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