1	chapter 6 chemical composition
2	"A ton of things have..." A ton of things have been synthesized; nylon, teflon, kevlar, pvc, nutrasweet… What new things are made of is very important In this chapter we will determine formulas BUT! first we must learn how to count atoms How???
3	6.1 counting by weighing How does one count candy, or pennies, or nails, or M&Ms, or recycled Aluminum cans??? they’re so numerous, and small! help! Count them by weighing them! (?) However, we must assume that all the “things” are 1) Identical OR 2) They have an average weight Since most things don’t occur identically, one must take an average of mass of those things…
4	"If you act as if..." If you act as if they are all that average weight, it’s easy: Take how much the whole pile weighs Divide it by the average mass of them ba-da-bing! you know how many there are! we do the same thing with tiny invisible atoms; we count by weighing!
5	6.2 Atomic masses: counting atoms by weighing OK, now we move to atoms Chemistry is wants to find out how many of these are needed to react with a certain number of those to get so many of these meaning: Numbers of things are Super -important time for thinking cap!
6	"The mass of an atom..." The mass of an atom is unbelievably small (e.g. a C atom weighs 1.99 x 10^-23 g) So kg and g are out! We need a new unit small enough to deal with these tiny guys reasonably Chemists use the atomic mass unit (amu) 1 amu = 1.66 X 10^-24 g atoms will weigh so many amu’s now back to atoms and counting by weighing…
7	"Remember isotopes?" Remember isotopes? not all atoms are the same?! Carbon exists as: C-12, C-13, and C-14 They occur in different types, but altogether they can give us an average mass The average atomic mass for carbon on this planet is 12.01 amu (all the rest are listed on the Periodic Table) Let’s act, for the rest of chemistry, as if atoms have this one mass, the one listed on the Periodic Table
8	Examples what is the mass, in amu, of 75 Al atoms? (1 Al = 26.98 amu)
9	More Examples how many atoms are in 1172.49 amu of Na atoms? (1 Na = 22.99 amu)
10	6.3 The mole Up to this point we’ve used submicroscopic amounts of stuff What about realistic amounts??? ready? How about making it easy for everyone?…
11	"How about we pick the..." How about we pick the number of atoms which will change amu’s to grams? What number will take… 12.01 amu C -> 12.01 g C 26.98 amu Al -> 26.98 g Al 63.55 amu Cu -> 63.55 g Just one number, Avogadro’s number, the number all chemists throughout the world use every day of their miserable lives, the one, the only..
12	"The mole 6.022 x 10²³" The mole 6.022 x 10²³
13	"1 dozen = 12 1..." 1 dozen = 12 1 ream = 500 1 pair = 2 1 gross = 144 1 mol = 6.022 x 10²³ Counting this number would take 2000 trillion years; a mole of sand could cover LA in 600 meters of sand; a mole of marbles would cover Earth in 50 miles of marbles; but! a mole of water you can cup in one hand! An element that weighs as much as the number of grams listed on the PeriodicTable has a mol, 6.022 x 10²³, atoms in it
14	"Now we can count atoms..." Now we can count atoms just by knowing: 1. how much we have (g) and 2. The number on the Periodic Table which now represents a mol of stuff in grams more than the molar mass means > 1 mol of stuff less than the molar mass means < 1 mol of stuff let’s start off easy…
15	Example (atoms-mol) How many atoms in 2.2 mol C?
16	Examples (g-mol) 26 g C = ? mol
17	Examples (g-mol-atoms) 25.0 g Ca = ? mol = ? atoms
18	Examples (g-mol-atoms) 5.00x 10²⁰ Cr atoms = ? mol = ? g
19	6.4 molar mass We dig further into the molar mass thing, and… converting moles into mass (visa versa) for compounds now see next the relationship between micro and macro
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21	"Do you want to find..." Do you want to find the molar mass of methane? (yes!) Just add up the little guys in the compound! for methane that would be: 12.01 (C) + 1.01(H) + 1.01(H) + 1.01(H) + 1.01(H) = 16.05 g/mol = molar mass Just like before: get me a mass of 16.05 g of methane and you’ve given me a mol of it
22	examples what is the molar mass of sulfur dioxide? (this is why you need to remember names/formulae) sulfur dioxide is SO₂ a mol of SO₂ has 1 mol S and 2 mol O = (1 x 32.07) + (2 x 16.00) = 64.07 g/mol
23	examples what is the molar mass of: water? H₂O = 18.02 g/mol ammonia? NH₃ = 17.03 g/mol propane, C₃H₈? = 44.09 g/mol glucose, C₆H₁₂O₆? = 180.2 g/mol
24	examples can do the same with ionic compounds! what is molar mass of: calcium sulfate? CaSO₄ = 136.3 g/mol sodium carbonate? Na₂CO₃ = 106.0 g/mol barium hydroxide? Ba(OH)₂ = 171.3 g/mol
25	examples (mol to mass) calculate molar mass of calcium carbonate what is the mass of 4.86 mol of this stuff? molar mass CaCO₃ = 100.09 g/mol
26	examples (mass to mol) calculate molar mass of juglone (C₁₀H₆O₃) what is the mol of 1.56 g of this stuff? molar mass C₁₀H₆O₃ = 174.1 g/mol
27	examples (mass to molecules) how many molecules of Teflon (C₂F₄) are in a 135-g sample? (hint: do you expect a little or gigantic answer?) think! g --> mol --> molecules you’ll need molar mass C₂F₄ = 100.02 g/mol
28	6.5 percent composition of compounds which score is better? 28/50 or 32/75? percent can answer the question percent is merely taking a part and dividing by total (then multiplying by 100) same with % comp… take mass contributed by one element and divide by total mass of cmpd (x 100)
29	"Can you tell the %..." Can you tell the % composition just from looking at a formula? e.g. is SO₂ 33% S and 67% O? NO! % composition is a gram ratio thing not a mole ratio thing!!! so first change the mole in the formula to grams, then find %…
30	example What is the % composition of SO₂? SO₂ weighs in at 64.07 g/mol S contributes 32.07 of it, oxygen 2 x 16.00 %S = (32.07/64.07) • 100 = 50.05% %O = (32.00/64.07) • 100 = 49.94%
31	example What is the % composition of C₁₀H₁₄O? C₁₀H₁₄O weighs in at 150.2 g/mol C contributes 120.1 of it, H gives it 14.11, and O 16.00 %C = (120.1/150.2) • 100 = 79.96% %H = (14.11/150.2) • 100 = 9.394% %O = (16.00/150.2) • 100 = 10.65%
32	example you have: a 36-g sample = 28 g Fe, 8 g O a 160-g sample = 112 g Fe, 48 g O are they the same substance? CAN’T TELL FROM JUST LOOKING AT GRAMS!!!! BUT, if they have the same % comp… ta da!
33	"for the 36-g sample =..." for the 36-g sample = 28 g Fe (28 g Fe / 36 g total ) • 100 = 78% Fe for the 160-g sample = 112 g Fe (112 g Fe / 160 g total ) • 100 = 70% Fe Not the same!
34	example #1: 45.0-g sample = 35.1 g Fe, 9.9 g O #2: 215.0-g sample = 167.7 g Fe, 47.3 g O are they the same? (35.1 g Fe / 45.0 g total ) • 100 = 78% Fe (167.7 g Fe / 215.0 g total ) • 100 = 78% Fe they are the same! (the oxygen percents will be the same, too)
35	example #1: 75.0-g sample = 20.5 g C, 54.5 g O #2: 135.0-g sample = 67.5 g C, 90.0 g O are they the same? (20.5 g C / 75.0 g total ) • 100 = 27.3% C (67.5g C / 135.0 g total ) • 100 = 50.0% C they are not the same!
36	6.6 Formulas of compounds 6.7 Calculation of empirical formulas smallest whole number ratio of a cmpd is empirical formula C₆H₆ : simplest formula = CH C₆H₁₂ : simplest formula = CH₂ C₃H₈ : simplest formula = C₃H₈ C₆H₁₂O₆ : simplest formula = CH₂O
37	"to determine the empirical formula..." to determine the empirical formula you must find MOLE RATIOS of elements in a compound!!! the easy way? just change % è g è mol
38	example 36-g iron oxide sample is 78% iron. What is the emperical form? (remember: think moles) 78% Fe è 78g Fe 22% O è 22g O 78g Fe è1.4 mol Fe 22g O è 1.4 mol O 1.4 mol Fe : 1.4 mol O is a 1:1 mol ratio Therefore, FeO
39	example 27-g sulfur oxide sample has 50% S. What is the emp form? 50% S è 50g S 50% O è 50g O 50g S è 1.56 mol S 50g O è 3.12 mol O 1.56 mol S : 3.12 mol O is a 1:2 mol ratio Therefore, SO₂
40	"If the ratio isn’t obvious" If the ratio isn’t obvious, divide mol of each substance by the smallest number…(See Page 178) e.g. what if the ratio ends up being something weird like 0.195 Fe:0.291 O ??? 0.195 / 0.195 = 1 0.291 / 0.195 = 1.5 This is a 1:1.5 ratio which is really 2:3 therefore Fe₂O₃
41	example 18.94 g Al reacts with O to make 35.74 g aluminum oxide. what is the empirical formula? (note: where is g O?) 18.94 g Al è 0.701 mol Al 16.80 g O è 1.05 mol O 0.701 / 0.701 = 1 Al 1.05 / 0.701 = 1.5 O This is a 2:3 ratio, therefore Al₂O₃
42	6.8 calculation of molecular formulas molecular formula is the real formula = a multiple of emp form molecular form = empirical form • n n = molecular mass / empirical mass
43	example The empirical formula for a cmpd is P₂O₅. The molecular mass is 283.88 g/mol. What is the molecular formula? emp mass = 141.94 g/mol divide 283.88 / 141.94 = 2 so, “multiply” P₂O₅ by two to get P₄O₁₀
44	big person example In a 32.0-g sample of hydrazine, there are 28.0 g N and 4.0 g H. The molar mass is 32.0 g/mol. What is the molecular formula? 1) find emp formula 28.0 g N è 2.0 mol N 4.0 g H è 4.0 mol H Emp formula = NH₂ 2) find molar mass ratios molar mass (32.0) / emp mass ( 16.0) = 2 3) multiply by 2 to get formula = N₂H₄
45	big person example 2 An additive for gasoline is 71.65% Cl, 24.27% C, 4.07% H. The molar mass is 98.96 g/mol. What is the molecular formula? 1) find emp formula 71.65 % Cl è 71.65 g Cl è 2.021 mol Cl 24.27 % C è 24.27 g C è 2.021 mol C 4.07 % H è 4.07 g H è 4.04 mol H Emp formula = ClCH₂ 2) find molar mass ratios molar mass (98.96) / emp mass (49.48) = 2 3) multiply by 2 to get formula = Cl₂C₂H₄