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The Mole Q: how long would it take to spend a mole of $1 coins if they were being spent at a rate of 1 billion per second?

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Background: atomic masses Look at the atomic masses on the periodic table. What do these represent? E.g. the atomic mass of C is 12 (atomic # is 6) We know there are 6 protons and 6 neutrons Protons and neutrons have roughly the same mass. So, C weighs 12 u (atomic mass units). What is the actual mass of a C atom? Answer: approx. 2 x grams (protons and neutrons each weigh about 1.7 x grams) Two problems 1.Atomic masses do not convert easily to grams 2.They cant be weighed (they are too small)

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The Mole With these problems, why use atomic mass at all? 1.Masses give information about # of p +, n 0, e – 2.It is useful to know relative mass E.g. Q - What ratio is needed to make H 2 O? A - 2:1 by atoms, but 2:16 by mass It is useful to associate atomic mass with a mass in grams. It has been found that 1 g H, 12 g C, or 23 g Na have 6.02 x atoms 6.02 x is a mole or Avogadros number mol is used in equations, mole is used in writing; one gram = 1 g, one mole = 1 mol. Read 4.3 (167-9). Stop after text beside fig 2. Do Q1-6. Challenge: 1 st slide (use reasonable units)

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The Mole: Answers 1.A mole is a number (like a dozen). Having this number of atoms allows us to easily convert atomic masses to molar masses x x 6.02 x = x or 1.81 x (note: there are 3 moles of atoms in one mole of CO 2 molecules. In other words, there are 5.42 x atoms in 3.00 mol CO 2 ) x a) 1.43 kg 12 = kg per orange b) 1.01 g 6.02 x = 1.68 x 10 –24 g

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Mollionaire Q: how long would it take to spend a mole of $1 coins if they were being spent at a rate of 1 billion per second? A: $ 6.02 x / $ = 6.02 x payments = 6.02 x seconds 6.02 x seconds / 60 = x minutes x minutes / 60 = x hours x hours / 24 = x 10 9 days x 10 9 days / = x 10 7 years A: It would take 19 million years

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Comparing sugar (C 12 H 22 O 11 ) & H 2 O No, sugar has more (45:3 ratio) Yes (6.02 x in each) Yes. No, molecules have dif. masses No, molecules have dif. sizes. 1 mol each Yes, thats what grams are. mass? No, they have dif. molar masses # of moles? No, they have dif. molar masses # of molecules? No # of atoms? No, they have dif. densities. volume? 1 gram eachSame

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Molar mass The mass of one mole is called molar mass E.g. 1 mol Li = 6.94 g Li This is expressed as 6.94 g/mol What are the following molar masses? SSO 2 Cu 3 (BO 3 ) g/mol g/mol g/mol Calculate molar masses (to 2 decimal places) CaCl 2 (NH 4 ) 2 CO 3 O 2 Pb 3 (PO 4 ) 2 C 6 H 12 O 6 Cux 3 = x 3= Bx 2 = x 2= Ox 6 = x 6=

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Molar mass The mass of one mole is called molar mass E.g. 1 mol Li = 6.94 g Li This is expressed as 6.94 g/mol What are the following molar masses? SSO 2 Cu 3 (BO 3 ) g/mol64.06 g/mol g/mol Calculate molar masses (to 2 decimal places) CaCl 2 (NH 4 ) 2 CO 3 O 2 Pb 3 (PO 4 ) 2 C 6 H 12 O g/mol (Ca x 1, Cl x 2) g/mol (N x 2, H x 8, C x 1, O x 3) g/mol (O x 2) g/mol (Pb x 3, P x 2, O x 8) g/mol (C x 6, H x 12, O x 6)

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Converting between grams and moles If we are given the # of grams of a compound we can determine the # of moles, & vise-versa In order to convert from one to the other you must first calculate molar mass g = mol x g/mol mol = g g/mol This can be represented in an equation triangle g molg/mol g= g/mol x mol0.25HCl 53.15H 2 SO NaCl 1.27Cu Equationmol (n)gg/molFormula mol= g g/mol g= g/mol x mol mol= g g/mol

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Simplest and molecular formulae Consider NaCl (ionic) vs. H 2 O 2 (covalent) Cl Na Cl Na Chemical formulas are either simplest (a.k.a. empirical) or molecular. Ionic compounds are always expressed as simplest formulas. Covalent compounds can either be molecular formulas (I.e. H 2 O 2 ) or simplest (e.g. HO) Q - Write simplest formulas for propene (C 3 H 6 ), C 2 H 2, glucose (C 6 H 12 O 6 ), octane (C 8 H 14 ) Q - Identify these as simplest formula, molecular formula, or both H 2 O, C 4 H 10, CH, NaCl H O O H H O O H H O O H

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Answers Q - Write simplest formulas for propene (C 3 H 6 ), C 2 H 2, glucose (C 6 H 12 O 6 ), octane (C 8 H 14 ) Q - Identify these as simplest formula, molecular formula, or both H 2 O, C 4 H 10, CH, NaCl A - CH 2 A - H 2 O is both simplest and molecular C 4 H 10 is molecular (C 2 H 5 would be simplest) CH is simplest (not molecular since CH cant form a molecule - recall Lewis diagrams) NaCl is simplest (its ionic, thus it doesnt form molecules; it has no molecular formula) CHCH 2 OC4H7C4H7 For more lessons, visit

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