Name:
Regents Chemistry: Mrs. Mintz
PracticePacket
Chapter10:ChemicalCalculations
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Objectives
• Describe how to calculate formula mass and gram
formula mass
• Distinguish between the atomic mass of an element
and its molar mass.
Gram Formula
Mass
Video Lesson 10.1
Stoichiometry
Quick Review
• What are the units we use for the mass of
atoms
o ANS: Atomic mass units (amu) or (u)
• The study of the quantitative relationships in
chemical reactions.
• What is the mass of one atom of oxygen?
o ANS: 15.9994 amu or u
Example 1: Calculating
Formula Mass
Gram Formula Mass
What is the formula mass of K2CO3?
• Formula mass
o The mass of an atom, molecule or compound in
ATOMIC MASS UNITS (amu) or (u)
o EX. Formula mass of a hydrogen atom is 1.00794
amu
K = 39.1 x 2 = 78.2
C = 12.0 x 1 = 12.0
O = 16.0 x 3 = 48.0
+
138.2 amu
• Gram Formula Mass
o The mass of one MOLE of an atom, molecule or
compound in GRAMS (g/mol)
o EX. GFM of hydrogen is 1.00794 g/mol
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Example 2: Calculating
Gram Formula Mass
Example 3: Calculating
Gram Formula Mass
What is the formula mass of K2CO3?
What is the gram formula mass of NH3?
K = 39.1 x 2 = 78.2
C = 12.0 x 1 = 12.0
O = 16.0 x 3 = +48.0
N = 1 x 14.0 = 14.0
H = 3 x 1.0 = +3.0
17.0 g/mol
138.2 g/mol
Example 4: Calculating
Gram Formula Mass
• What is the gram formula mass of Mg(OH)2
Mg = 1 x 24.3 = 24.3
O = 2 x 16.0 = 32.0
H = 2 x 1.0 = 2.0
+
58.3 g/mol
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Objective:
Percent
Composition
How do we calculate the percent
composition by mass for a hydrate or
anhydrous using the percent composition
formula?
Chemistry 200
Video Lesson 10.2
Hydrate -
Percent Composition
H2O molecule(s) a<ached to an ionic crystal. Imagine a big bucket w/ 100 pieces of fruit in it. If the bucket has 25 apples, 30 oranges, 20 pears & 25 bananas, what percentage of the bucket consists of pears?
We calculate the gram formula mass as a separate unit.
ex: CaSO4•2H2O
Anhydrous -ionic substances without
How do we determine the difference
between a Hydrate and Anhydrous?
20 pears X
100
= 20%
100 pieces
H2O
Percent Composition can also be applied to the masses of elements in a compound using the following formula:
% composition by mass =
mass of part x 100
mass of whole
What is the % of water in Na2Cr2O7•10H2O
Na =23.0 g/mole x 2 atoms --> 46.0g/mol
Cr = 52.0 g/mole x 2 atoms --> 104.0g/mol
O =16.0 g/mole x 7 atoms --> 112.0g/mol
H2O =18.0 g/mole x 10 atoms à 180.0g/mol
442.0 g/mol
ex: What % of oxygen is in KClO3? GFM àK 1 atom 1 x 39.1 g/mol = 39.1
Cl 1 atom 1 x 35.5 g/mol = 35.5
O 3 atoms 3 x 16.0 g/mol = 48.0
122.6 g/mol
% of H2O = 180.0g/molx 100 =40.7 % H2O
442.0 g/mol
% Oxygen = 48.0X 100 =39.2%
122.6
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Objectives
• Define Avogadro’s number as it relates to a mole of
a substance.
• Describe how the mass of a mole of a compound is
calculated.
The Mole
Video Lesson 10.3
• A mole is a chemistry unit of measure
6.02 x 1023 atoms of C or 1 mole
12 bagels or 1 dozen
How many moles are in 39.0 grams of LiF?
Li = 1 x 7.0
F = 1 x 19.0
= 7.0
= 19.0
26.0 g/mol
: Solve for Moles
Moles =
39.0 g
26.0 g/mol
Moles = 1.50 mol
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What is the mass of 4.50 moles of KOH?
K = 1 x 39.1 = 39.1
O = 1 x 16.0 = 16.0
H = 1 x 1.0 = +1.0
56.1 g/ mol
4.50 mol =
g
56.1 g/mol
g = 252.5g 2
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Objective:
Mole to Mole
Ratios
How do we determine a new mole ratio in
a reaction when the molar amount of a
reactant or product is changed?
Chemistry 200
Video Lesson 10.4
Mole to Mole ratios
20 Cookies =
1cup-flour, 4 eggs, 2 tsp-salt, 3oz-bu>er, 12oz-choc chips
In problems involving chemical reactions, the relative
amounts of reactants & products are represented by
the coefficients. Coefficients represent both the basic
unit & mole ratios in balanced equations. The ratio is
like a recipe for the reaction. The ratio tells how
many moles of each species are necessary for the
balanced reaction
30 Cookies =
1.5cup-flour, 6 eggs, 3 tsp-salt, 4.5oz-bu>er, 18oz-choc chips
10 Cookies =
.5 cup-flour, 2 eggs, 1 tsp-salt, 1.5oz-bu>er, 6oz-choc chips
We can use the Mole Ratios btwn elements & molecules in a
chemical equation to make predictions about how they will
react in nature by applying the following procedure:
**Ratios must be moles to moles, NOT grams to grams or grams to moles**
Combustion of butane
2 C4H10(g) + 13 O2(g) --> 8 CO2(g) + 10 H2O(g)
3.55 moles of C4H10 will yield how many moles of CO2?
3.55 moles C4H10= x moles CO2
8 moles CO2
2 moles C4H10
1. Put boxes around the 2 substances that are referred to in the question
2. Set up a proportion using the mole ratios of the 2
substances:
Solve for x -->cross multiply
a. Place the numbers in the chemical equation on the bo>om
part of the ratio
2(x) = 8(3.55)
b. Place the numbers in the word problem
on the top of the ratio
x = 28.4 2
x = 14.2 moles of CO2
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4Al + 3O2 --> 2Al2O3
What information do we know about the above equation?
- Aluminum, Oxygen & Aluminum Oxide
Qualitative
-  4 moles of Aluminum, 3 moles of Oxygen Quantitative
& 2 moles of Aluminum Oxide
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Objective:
Empirical &
Molecular
Formulas
How do we determine the molecular
formula for a substance if given the
empirical formula and gram formula mass?
Chemistry 200
Video Lesson 10.5
Molecular Formula – A molecular formula represents
the true formula of a substance. Unlike ionic formulas
that need to be simplified, a molecular compound like
dinitrogen tetroxide (N2O4) would not be simplified
because that is the molecular formula. Other examples would include N2O, CO2, C3H6, etc. If
we were to represent these examples as empirical
formulas, C3H6 could be simplified while the others are already empirical formulas.
Empirical & Molecular Formulas
Empirical Formula – The lowest whole number
ratio of atoms in a compound. For example, C6H6
would have an empirical formula of CH. C6H12O6
would have an empirical formula of CH2O. An
empirical formula can also be the same as the
molecular formula, as in the case of H2O.
Determining an empirical formula from a
molecular formula
Determining a molecular formula from an
empirical formula To convert a molecular formula into an empirical
formula:
• Find the largest common multiple of all the
subscripts in a formula
• Divide each subscript by that common multiple to
create an empirical formula.
• Converting an empirical formula into a molecular
formula is a liUle trickier since we do not know the
common multiple.
• Consider the empirical formula CH2. This
empirical formula can yield many
molecular formulas including CH2,
C2H4, C3H6, C4H8 etc.
C6H9O3
3 = C2H3O
C6H14
2 = C2H7
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To convert an empirical formula into a
molecular formula, use the following steps: Determine the molecular formula of a substance that has a molecular mass of 84.0 g/mole & an empirical formula of CH2
1.  Determine the molecular mass of the empirical
formula.
Molecular mass of CH2 (empirical formula)
C --> 1 atom x 12.0 g/mole = 12.0 g/mole
2.  Divide the molecular mass (GFM) of the molecular
formula by the molecular mass of the empirical
formula. This represents the common multiple.
H --> 2 atoms x 1.0 g/mole = 2.0 g/mole
14.0 g/mole
Molecular mass of molecular formula
Molecular mass of empirical formula
3.  Multiply each subscript in the
empirical formula by this
common multiple to obtain
the molecular formula.
84.0 g/mole = 6
14.0 g/mole
Molecular formula =
CH2 x 6 =C6H12
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PracticePacket:ChemicalCalculations
Video10.1:GramFormulaMass
Chemical Formula
1. MgBr 2
Gram Formula M ass
2. KCl
3. FeCl 2
4. CrF 2
5. Al 2 S 3
6. PbO
7. TiI 4
8. Mg 3 P 2
9. SnCl 2
10. HgCl 2
11. H 2 SO 4
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PracticePacket:ChemicalCalculations
Video10.2:PercentComposition
Completethefollowingcalculations.Showallworkandunits.RoundGFMtonearesttenth.
1. WhatisthepercentbymassofmagnesiuminMg3(PO4)2?
2. WhatpercentageofNa2SO4isoxygenbymass?
3. Whatisthepercentofnitrogenbymassinammoniumnitrate?
4. WhatisthepercentbymassofwaterinAl(ClO3)3Ÿ6H2O?
5. Calculatethe%ofhydrogeninnitroglycerine{C3H5N3O9}.
6. WhatpercentageofCaSO4Ÿ2H2Oiswaterbymass?
7. Calculatethepercentcompositionofoxygeninglycerol{C3H5(OH)3}.
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PracticePacket:ChemicalCalculations
8. Calculatethepercentaluminuminaluminumoxide.
9. Calculatethepercentofphosphorusinsodiumphosphate?
10. CalculatethepercentbymassofwaterinCuNO3Ÿ5H2O?
Video10.3:TheMole
Usingthemoleformulaconvertthefollowingfromgramstomoles.Anexampleisprovided.
Roundtothenearesthundredth.
Example:Convert15.0gofCaCl2tomoles.
Step1:CalculategramStep2:Plugintomoleequation
formulamassmoles=grams/GFM
Ca=1x40.08=40.08moles=15.0g/110.98g
Cl=2x35.45=70.90moles=0.140moles
110.98grams/mol
1.25.0gNaOH
2.54.0gofHCl
3.375.0gHNO3
4.46.0g(NH4)2CO3
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PracticePacket:ChemicalCalculations
Usingthemoleformulaconvertthefollowingfrommolestograms.Anexampleisprovided.
Roundtothenearesthundredth.
Example:Convert2.5molesofCaCl2tograms
Step1:CalculategramStep2:Plugintomoleequation
formulamassmoles=grams/GFM
Ca=1x40.08=40.082.5moles=g/110.98g
Cl=2x35.45=70.90grams=277.5grams
110.98grams/mol
1.2.0molsofH2SO4
3.1.0molsFe2O3
2.4.5molsofN2I6
4.2.5molsC2H5OH
Video10.4:MoletoMoleRatios
Determinethemolaramountusingmoletomoleratiosforthefollowingreactions
1. Giventhereaction:4NH3+5O2à4NO+6H2O
WhatisthemaximumnumberofmolesofH2Othatcanbeproducedwhen2.0moles
ofNH3arecompletelyreacted?
2. Giventhereaction:
2KClO3à2KCl+3O2
Whatisthetotalnumberofmolesofpotassiumchlorateneededtoproduce6.0
molesofO2?
3. Giventhereaction:
2C2H2+5O2à4CO2+2H2O
Howmanymolesofoxygenarerequiredtoreactcompletelywith1.0molesofC2H2?
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Moles
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PracticePacket:ChemicalCalculations
4. Giventhereaction:Fe2O3+3COà2Fe+3CO2
Howmanymolesofcarbondioxideareproducedwhen0.5molesofFe2O3are
completelyreacted?
ConvertyouranswertogramsofCO2
5. Giventhereaction:
3MgCl2+2Alà3Mg+2AlCl3
Howmanymolesofaluminumchlorideareproducedwhen8molesofmagnesium
chloridearecompletelyreactedwithaluminum?
6. Considerthecombustionofmethane.Howmanymolesofcarbondioxideareobtained
when20molesofmethanearecompletelyburned.
7. Giventhereaction:
2H2+O2à2H2O
HowmanymolesofH2Ocanbeproducedif20molesofhydrogencompletelyreact
withoxygen?
ConservationofMass
Achemicalequationisasetofsymbolsthatstatetheproductsandreactantsina
chemicalreaction.TheLawofConservationofMassstatesthatmasscanneitherbecreated
nordestroyedinachemicalreaction.Thisgoesformoleamountsasinbalancingequations
andmassamounts.
Example:Giventhereaction:N2+3H2à2NH3
WhatisthetotalnumberofgramsofH2thatreactswhen14gramsofN2arecompletely
consumedtoproduce20gramsofNH3?
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LawofConservationofMassWorksheet
Since there is a conservation of mass in all balanced chemical reactions, these problems are also quite easy.
Consider the following balanced chemical reaction CaCO3 à CaO + CO2. In this reaction the mass of
CaCO3 must equal the combined mass of CaO and CO2. If you know the mass of the reactants you
automatically know the mass of the products and vice-versa.
Using the reaction above, determine the mass of CaO produced if 200 grams of CaCO3 decomposed and
produced 88 grams of CO2.
CaCO3 à CaO + CO2
200g = Xg + 88g à 200g – 88g = 112g CaO produced
Complete the following problems
1) Hydrogenandoxygenreactchemicallytoformwater.Howmuchwaterwouldformif14.8grams
ofhydrogenreactedwith34.8gramsofoxygen?(H2+O2àH2O)
2) Whenammoniumnitrate(NH4NO3)explodes,theproductsarenitrogen,oxygen,andwater.When
40gramsofammoniumnitrateexplode,14gramsofnitrogenand8gramsofoxygenform.How
manygramsofwaterform?(NH4NO3àN2+O2+H2O)
3) 40gofcalciumreactswith71gofchlorinetoproduce_____gofcalciumchloride.
4) _____gofpotassiumreactswith16gofoxygentoproduce94gofpotassiumoxide.
5) 14goflithiumreactionwith_____gsulfurtoproduce46goflithiumsulfide.
6) 24gofmagnesiumreactswith38goffluorinetoproduce_____gmagnesiumfluoride.
7) 65.5gcopperreactswith_____goxygentoproduce81gcopper(I)oxide.
8) 88gofstrontiumreactswith160gbrominetoproduce_____gstrontiumbromide.
9) 46gofsodiumreactswith_____gchlorinetoproduce117gsodiumchloride.
10)____gironreactswith71gchlorinetoproduce129gofiron(II)chloride.
11)137gofbariumreactswith______giodinetoproduce391gbariumiodide.
12)_____ghydrogenreactswith32gofoxygentoproduce34gofhydrogenperoxide.
13)Whydowebalancechemicalreactions?
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PracticePacket:ChemicalCalculations
Practice:
1. Giventhebalancedequationrepresentingareaction:
CaO(s)+CO2(g)àCaCO3(s)+heat
WhatisthetotalmassofCaOthatreactswith75gramsofCO2(g)toproduce200.
GramsofCaCO3(s)?
2. Giventhebalancedequationrepresentingareaction:
2H2+O2à2H2O
Whatisthetotalmassofwaterformedwhen4gramsofhydrogenreactscompletely
with32gramsofoxygen?
Video10.5:EmpiricalandMolecularFormulas
Determininganempiricalformulafromamolecularformula
Toconvertamolecularformulaintoanempiricalformula,weneedtofindthelargest
commonmultipleofallthesubscriptsinaformula.Thensimplydivideeachsubscriptby
thatcommonmultipletocreateanempiricalformula.
MolecularFormula
LargestCommon
Multiple
EmpiricalFormula
C6H9O3
C6H14
C2H4O2
H2O2
C12H22
C12H24
N2O3
C3 H6 3
C2H3O
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PracticePacket:ChemicalCalculations
Determiningamolecularformulafromanempiricalformula
1. Determinethemolecularmassoftheempiricalformula.
2. Dividethemolecularmassofthemolecularformulabythemolecularmassofthe
empiricalformula.Thisrepresentsthecommonmultiple.
3. Multiplyeachsubscriptintheempiricalformulabythiscommonmultipletoobtainthe
molecularformula.
Practice
1. Asubstancew/amolecularmassof172.0g/moleisdeterminedtohaveanempirical
formulaofC3H7.Determinethemolecularformulaofthissubstance.
2. Determinethemolecularformulaofasubstancew/anempiricalformulaofC2H3S2and
amolecularmassof456.0g/mole.
3. Determinethemolecularformulaofasubstancew/anempiricalformulaofC2H4N
andamolecularmassof294.0g/mole.
4. DeterminethemolecularformulaofacompoundwithanempiricalformulaofNH2and
amolecularmassof32.0g/mole.
5. EthanegashastheempiricalformulaCH3andamolecularmassof165.0g/mole.
Determinethemolecularformula.
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PracticePacket:ChemicalCalculations
6. VitaminChasanempiricalformulaofC3H4O3andamolecularmassof264.0g/mole.
Determinethemolecularformula.
7. Ibuprofen,acommonheadacheremedy,hasanempiricalformulaofC7H9Oandamolar
massof545.0g/mole.Determinethemolecularformula.
8. OxalicacidhastheempiricalformulaHCO2andamolarmassof90.0g/mole.
Determinethemolecularformula.
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Chemical Calculations Review
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PracticePacket:ChemicalCalculations
FreeResponseReview
1. Considerthereactionbelowthatrepresentsthecombustionofbenzene,C6H6
2C2H6+15O2à12CO2+6H2O
a. Howmanymolesofbenzenearein22.3gramsampleofbenzene?
b. Whatisthemassofa1.79molesampleofO2?
c. A4.4molesampleofO2willreacttoproducehowmanymolesofCO2?
d. Howmanymolesofbenzeneareneededtoreactcompletelywith3.47molesofO2?
2. Putrescine,isacompoundproducedindecayinganimalsandcreatesthecharacteristic
odorassociatedwithrottenmeat.TheempiricalformulaforputrescineisC2H6N.
a. Ifputrescinehasamolecularmassof88.0g/mol,determinethemolecularformula
ofputrescine.
b. Howmanymolesofputrescineareina37.5gramsampleofputrescine?
c. Whatisthepercentofnitrogeninputrescine?
3. Answerthefollowingquestionsaboutthefollowinghydratedsalt:
a. CalculatethegramformulamassofCaSO3•4H2O
b. DeterminethemolesofCaSO3•4H2Oina15.6gramsample.
c. CalculatethepercentofwaterinCaSO3•4H2O
4. Inanexperiment,4.56gramsofsodiumcompletelyreactswithsulfur,producing7.89
gramsofsodiumsulfide.Determinethetotalmassofsulfurconsumed.
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