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Chapter 8 Chapter 8

Chemical CompositionChemical Composition

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How you measure how much?How you measure how much? You can measure mass, You can measure mass, or volume,or volume, or you can count pieces.or you can count pieces. We measure mass in grams.We measure mass in grams. We measure volume in liters.We measure volume in liters.

We count pieces in We count pieces in MOLES.MOLES.

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A. What is the Mole?A. What is the Mole?

A counting number (like a dozen)A counting number (like a dozen)

Avogadro’s number (NAvogadro’s number (NAA))

1 mol = 1 mol = 6.02 6.02 10 102323 items items

A large amount!!!!A large amount!!!!

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1 mole of hockey pucks would 1 mole of hockey pucks would equal the mass of the moon!equal the mass of the moon!

What is the Mole?What is the Mole?

1 mole of pennies would cover 1 mole of pennies would cover the Earth 1/4 mile deep!the Earth 1/4 mile deep!

1 mole of basketballs would fill a 1 mole of basketballs would fill a bag the size of the earth!bag the size of the earth!

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MolesMoles Defined as the number of carbon Defined as the number of carbon

atoms in exactly 12 grams of carbon-atoms in exactly 12 grams of carbon-12.12.

1 mole is 6.02 x 101 mole is 6.02 x 102323 particles. particles. Treat it like a very large dozenTreat it like a very large dozen 6.02 x 106.02 x 102323 is called Avagadro’s is called Avagadro’s

number.number.

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Representative particlesRepresentative particles The smallest pieces of a substance.The smallest pieces of a substance. For a molecular compound it is a For a molecular compound it is a

molecule.molecule. For an ionic compound it is a For an ionic compound it is a

formula unit.formula unit. For an element it is an atom.For an element it is an atom.

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Types of questionsTypes of questions How many oxygen atoms in the How many oxygen atoms in the

following?following?

– CaCOCaCO33

– AlAl22(SO(SO44))33 How many ions in the following?How many ions in the following?

– CaClCaCl22– NaOHNaOH

– AlAl22(SO(SO44))33

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Molar MassMolar Mass Mass of 1 mole of an element or compound.Mass of 1 mole of an element or compound.

Atomic mass tells the...Atomic mass tells the...– atomic mass units per atom (amu)atomic mass units per atom (amu)– grams per mole (g/mol)grams per mole (g/mol)

Round to 2 decimal placesRound to 2 decimal places

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Molar Mass ExamplesMolar Mass Examples

carboncarbon

aluminumaluminum

zinczinc

12.01 g/mol12.01 g/mol

26.98 g/mol26.98 g/mol

65.39 g/mol65.39 g/mol

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Molar Mass ExamplesMolar Mass Examples

waterwater

sodium chloridesodium chloride

– HH22OO

– 2(1.01) + 16.00 = 18.02 g/mol2(1.01) + 16.00 = 18.02 g/mol

– NaClNaCl– 22.99 + 35.45 = 58.44 g/mol22.99 + 35.45 = 58.44 g/mol

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Molar Mass ExamplesMolar Mass Examples

sodium bicarbonatesodium bicarbonate

sucrosesucrose

– NaHCONaHCO33

– 22.99 + 1.01 + 12.01 + 3(16.00) 22.99 + 1.01 + 12.01 + 3(16.00)

= 84.01 g/mol= 84.01 g/mol

– CC1212HH2222OO1111

– 12(12.01) + 22(1.01) + 11(16.00) 12(12.01) + 22(1.01) + 11(16.00)

= 342.34 g/mol= 342.34 g/mol

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Molar ConversionsMolar Conversions

molar mass

(g/mol)

MASS

IN

GRAMS

MOLESNUMBER

OF

PARTICLES

6.02 1023

(particles/mol)

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Molar Conversion ExamplesMolar Conversion Examples How many moles of carbon are How many moles of carbon are

in 26 g of carbon? in 26 g of carbon?

26 g C26 g C 1 mol C1 mol C

12.01 g C12.01 g C= 2.2 mol C= 2.2 mol C

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Molar Conversion ExamplesMolar Conversion Examples How many molecules are in How many molecules are in

2.50 moles of C2.50 moles of C1212HH2222OO1111? ?

2.50 mol2.50 mol6.02 6.02 10 102323

moleculesmolecules

1 mol1 mol= 1.51 = 1.51 10 102424

moleculesmolecules CC1212HH2222OO1111

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Molar Conversion ExamplesMolar Conversion Examples Find the mass of 2.1 Find the mass of 2.1 10 102424

molecules of NaHCOmolecules of NaHCO33. .

2.1 2.1 10 102424

moleculesmolecules 1 mol1 mol

6.02 6.02 10 102323

moleculesmolecules

= 290 g NaHCO= 290 g NaHCO33

84.01 g84.01 g

1 mol1 mol

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Types of questionsTypes of questions How many molecules of COHow many molecules of CO22 are the in are the in

4.56 moles of CO4.56 moles of CO22 ? ?

How many moles of water is 5.87 x 10How many moles of water is 5.87 x 102222 molecules?molecules?

How many atoms of carbon are there in How many atoms of carbon are there in 1.23 moles of C1.23 moles of C66HH1212OO66 ? ?

How many moles is 7.78 x 10How many moles is 7.78 x 102424 formula formula units of MgClunits of MgCl22??

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ExamplesExamples How much would 2.34 moles of How much would 2.34 moles of

carbon weigh?carbon weigh? How many moles of magnesium in How many moles of magnesium in

24.31 g of Mg?24.31 g of Mg? How many atoms of lithium in 1.00 g How many atoms of lithium in 1.00 g

of Li?of Li? How much would 3.45 x 10How much would 3.45 x 102222 atoms atoms

of U weigh?of U weigh?

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What about compounds?What about compounds? in 1 mole of Hin 1 mole of H22O molecules there are two O molecules there are two

moles of H atoms and 1 mole of O atomsmoles of H atoms and 1 mole of O atoms To find the mass of one mole of a To find the mass of one mole of a

compound compound – determine the moles of the elements determine the moles of the elements

they havethey have– Find out how much they would weighFind out how much they would weigh– add them upadd them up

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What about compounds?What about compounds? What is the mass of one mole of CHWhat is the mass of one mole of CH44??

1 mole of C = 12.01 g1 mole of C = 12.01 g 4 mole of H x 1.01 g = 4.04g4 mole of H x 1.01 g = 4.04g 1 mole CH1 mole CH44 = 12.01 + 4.04 = 16.05g = 12.01 + 4.04 = 16.05g

The The Gram Molecular mass Gram Molecular mass of CHof CH44 is is

16.05g16.05g The mass of one mole of a molecular The mass of one mole of a molecular

compound.compound.

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Gram Formula MassGram Formula Mass The mass of one mole of an ionic The mass of one mole of an ionic

compound.compound. Calculated the same way.Calculated the same way.

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Molar MassMolar Mass The generic term for the mass of one The generic term for the mass of one

mole. mole. The same as gram molecular mass, The same as gram molecular mass,

gram formula mass, and gram atomic gram formula mass, and gram atomic mass.mass.

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ExamplesExamples Calculate the molar mass of the Calculate the molar mass of the

following and tell me what type it is.following and tell me what type it is. NaNa22SS

NN22OO44

CC

Ca(NOCa(NO33))22

CC66HH1212OO66

(NH(NH44))33POPO44

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Using Molar MassUsing Molar Mass

Finding moles of compoundsFinding moles of compounds

Counting pieces by weighingCounting pieces by weighing

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Molar MassMolar Mass The number of grams of 1 mole of The number of grams of 1 mole of

atoms, ions, or molecules.atoms, ions, or molecules. We can make conversion factors We can make conversion factors

from these.from these. To change grams of a compound to To change grams of a compound to

moles of a compound.moles of a compound.

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ExamplesExamples How many moles is 4.56 g of COHow many moles is 4.56 g of CO22 ? ?

How many grams is 9.87 moles of How many grams is 9.87 moles of HH22O?O?

How many molecules in 6.8 g of How many molecules in 6.8 g of CHCH44??

49 molecules of C49 molecules of C66HH1212OO66 weighs how weighs how

much?much?

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Gases and the MoleGases and the Mole

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GasesGases Many of the chemicals we deal with Many of the chemicals we deal with

are gases.are gases. They are difficult to weigh.They are difficult to weigh. Need to know how many moles of Need to know how many moles of

gas we have.gas we have. Two things effect the volume of a gasTwo things effect the volume of a gas Temperature and pressureTemperature and pressure Compare at the same temp. and Compare at the same temp. and

pressure.pressure.

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Standard Temperature and Standard Temperature and PressurePressure

0ºC and 1 atm pressure0ºC and 1 atm pressure abbreviated STPabbreviated STP At STP 1 mole of gas occupies 22.4 LAt STP 1 mole of gas occupies 22.4 L Called the molar volumeCalled the molar volume Avagadro’s Hypothesis - at the same Avagadro’s Hypothesis - at the same

temperature and pressure equal temperature and pressure equal volumes of gas have the same number volumes of gas have the same number of particles.of particles.

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ExamplesExamples What is the volume of 4.59 mole of What is the volume of 4.59 mole of

COCO22 gas at STP? gas at STP?

How many moles is 5.67 L of OHow many moles is 5.67 L of O2 2 at at

STP?STP? What is the volume of 8.8g of CHWhat is the volume of 8.8g of CH44

gas at STP? gas at STP?

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Density of a gasDensity of a gas D = m /VD = m /V for a gas the units will be g / Lfor a gas the units will be g / L We can determine the density of any We can determine the density of any

gas at STP if we know its formula.gas at STP if we know its formula. To find the density we need the mass To find the density we need the mass

and the volume.and the volume. If you assume you have 1 mole than If you assume you have 1 mole than

the mass is the molar mass (PT)the mass is the molar mass (PT) At STP the volume is 22.4 L.At STP the volume is 22.4 L.

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ExamplesExamples Find the density of COFind the density of CO22 at STP.at STP.

Find the density of CHFind the density of CH44 at STP. at STP.

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The other wayThe other way Given the density, we can find the molar Given the density, we can find the molar

mass of the gas.mass of the gas. Again, pretend you have a mole at STP, Again, pretend you have a mole at STP,

so V = 22.4 L.so V = 22.4 L. m = D x Vm = D x V m is the mass of 1 mole, since you have m is the mass of 1 mole, since you have

22.4 L of the stuff.22.4 L of the stuff. What is the molar mass of a gas with a What is the molar mass of a gas with a

density of 1.964 g/L?density of 1.964 g/L? 2.86 g/L?2.86 g/L?

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All the things we can changeAll the things we can change

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We have learned how to We have learned how to change moles to gramschange moles to grams moles to atomsmoles to atoms moles to formula unitsmoles to formula units moles to moleculesmoles to molecules moles to litersmoles to liters molecules to atomsmolecules to atoms formula units to atomsformula units to atoms formula units to ionsformula units to ions

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Moles

Mass

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Moles

MassPT

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Moles

MassVolume PT

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Moles

MassVolume PT22.4 L

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Moles

MassVolume

Representative Particles

PT22.4 L

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6.02 x 1023

Moles

MassVolume

Representative Particles

PT22.4 L

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Moles

MassVolume

Representative Particles

6.02 x 1023

PT

Atoms

22.4 L

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Moles

MassVolume

Representative Particles

6.02 x 1023

PT

Atoms Ions

22.4 L

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Percentage CompositionPercentage Composition the percentage by mass of each the percentage by mass of each

element in a compoundelement in a compound

100mass total

element of massncompositio %

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100 =100 =

Percentage CompositionPercentage Composition

%Cu =%Cu =127.10 g Cu127.10 g Cu

159.17 g Cu159.17 g Cu22SS 100 =100 =

%S =%S =32.07 g S32.07 g S

159.17 g Cu159.17 g Cu22S S

79.852% Cu79.852% Cu

20.15% S20.15% S

Find the % composition of CuFind the % composition of Cu22S. S.

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%Fe =%Fe =28 g28 g

36 g 36 g 100 =100 = 78% Fe78% Fe

%O =%O =8.0 g8.0 g

36 g 36 g 100 =100 = 22% O22% O

Find the percentage composition Find the percentage composition of a sample that is 28 g Fe and of a sample that is 28 g Fe and 8.0 g O.8.0 g O.

Percentage CompositionPercentage Composition

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How many grams of copper are How many grams of copper are in a 38.0-gram sample of Cuin a 38.0-gram sample of Cu22S?S?

(38.0 g Cu(38.0 g Cu22S)(0.79852) = S)(0.79852) = 30.3 g Cu30.3 g Cu

CuCu22S is 79.852% Cu S is 79.852% Cu

Percentage CompositionPercentage Composition

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100 =100 =%H%H22O =O = 36.04 g36.04 g

147.02 g 147.02 g 24.51%24.51%

HH22OO

Find the mass percentage of Find the mass percentage of water in calcium chloride water in calcium chloride dihydrate, CaCldihydrate, CaCl22•2H•2H22OO??

Percentage CompositionPercentage Composition

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The Empirical FormulaThe Empirical Formula The lowest whole number ratio of The lowest whole number ratio of

elements in a compound.elements in a compound. The molecular formula the actual The molecular formula the actual

ration of elements in a compound.ration of elements in a compound. The two can be the same. The two can be the same. CHCH22 empirical formula empirical formula CC22HH44 molecular formula molecular formula CC33HH66 molecular formula molecular formula HH22O bothO both

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Calculating EmpiricalCalculating Empirical Just find the lowest whole number ratioJust find the lowest whole number ratio CC66HH1212OO66 CHCH44NN It is not just the ratio of atoms, it is also It is not just the ratio of atoms, it is also

the ratio of moles of atoms.the ratio of moles of atoms. In 1 mole of COIn 1 mole of CO22 there is 1 mole of there is 1 mole of

carbon and 2 moles of oxygen.carbon and 2 moles of oxygen. In one molecule of COIn one molecule of CO22 there is 1 atom there is 1 atom

of C and 2 atoms of O. of C and 2 atoms of O.

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Empirical FormulaEmpirical Formula

CC22HH66

CHCH33

reduce subscripts

Smallest whole number ratio of Smallest whole number ratio of atoms in a compoundatoms in a compound

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Calculating EmpiricalCalculating Empirical1.1. Means we can get ratio from percent composition.Means we can get ratio from percent composition.

2.2. Assume you have a 100 g. The percentages become grams.Assume you have a 100 g. The percentages become grams.

3.3. Find moles of each element.Find moles of each element.

4.4. Divide moles by the smallest # to find subscripts.Divide moles by the smallest # to find subscripts.

5.5. When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s.When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s.

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Empirical FormulaEmpirical Formula Find the empirical formula for a Find the empirical formula for a

sample of 25.9% N and 74.1% O. sample of 25.9% N and 74.1% O.

25.9 g25.9 g 1 mol1 mol

14.01 g 14.01 g = 1.85 mol N= 1.85 mol N

74.1 g74.1 g 1 mol1 mol

16.00 g 16.00 g = 4.63 mol O= 4.63 mol O

1.85 mol1.85 mol

1.85 mol1.85 mol

= 1 N= 1 N

= 2.5 O= 2.5 O

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Empirical FormulaEmpirical Formula

NN11OO2.52.5

Need to make the subscripts Need to make the subscripts whole numberswhole numbers multiply by 2multiply by 2

NN22OO55

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Molecular FormulaMolecular Formula ““True Formula” - the actual True Formula” - the actual

number of atoms in a compoundnumber of atoms in a compound

CHCH33

CC22HH66

empiricalempiricalformulaformula

molecularmolecularformulaformula

?

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Molecular FormulaMolecular Formula1. Find the empirical formula.1. Find the empirical formula.2. Find the empirical formula mass.2. Find the empirical formula mass.3. Divide the molecular mass by the empirical mass.3. Divide the molecular mass by the empirical mass.4. Multiply each subscript by the answer from step 3.4. Multiply each subscript by the answer from step 3.

nmass EF

mass MF nEF

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Molecular FormulaMolecular Formula The empirical formula for ethylene is CHThe empirical formula for ethylene is CH22. Find the . Find the

molecular formula if the molecular mass is molecular formula if the molecular mass is 28.1 g/mol? 28.1 g/mol?

28.1 g/mol28.1 g/mol

14.03 g/mol 14.03 g/mol = 2.00= 2.00

empirical mass = 14.03 g/molempirical mass = 14.03 g/mol

(CH(CH22))2 2 C C22HH44

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ExampleExample Calculate the percent composition of Calculate the percent composition of

a compound that is 29.0 g of Ag with a compound that is 29.0 g of Ag with 4.30 g of S.4.30 g of S.

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ExamplesExamples Calculate the percent composittion Calculate the percent composittion

of Cof C22HH44??

Aluminum carbonate.Aluminum carbonate.

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ExampleExample Calculate the empirical formula of a compound Calculate the empirical formula of a compound

composed of 38.67 % C, 16.22 % H, and 45.11 composed of 38.67 % C, 16.22 % H, and 45.11 %N.%N.

A compound is 43.64 % P and 56.36 % O. What A compound is 43.64 % P and 56.36 % O. What is the empirical formula?is the empirical formula?

Caffeine is 49.48% C, 5.15% H, 28.87% N and Caffeine is 49.48% C, 5.15% H, 28.87% N and 16.49% O. What is its empirical formula?16.49% O. What is its empirical formula?

A compound is known to be composed of A compound is known to be composed of 71.65 % Cl, 24.27% C and 4.07% H. Its molar 71.65 % Cl, 24.27% C and 4.07% H. Its molar mas is known (from gas density) is known to mas is known (from gas density) is known to be 98.96 g. What is its molecular formula?be 98.96 g. What is its molecular formula?