Chapter 3 1

StoichiometryStoichiometry

Chapter 3Chapter 3

Mrs. WestonMrs. Weston

Seneca Valley SHSSeneca Valley SHS

Chapter 3 2

• Stoichiometry - The study of quantities of materials consumed and produced in chemical reactions.

• Stoichiometric coefficients: numbers in front of the chemical formulas; give ratio of reactants and products.

Chemical EquationsChemical Equations

Chapter 3 3

•Which of the following correctly describes the balanced chemical equation given below? There may be more than one true statement. If a statement is incorrect, explain what is incorrect about it.

•4Al + 3O2 2Al2O3

I. For every 4 atoms of aluminum that react with 6 atoms of oxygen, 2 molecules of aluminum oxide are produced.

II. For every 4 moles of aluminum that react with 3 moles of oxygen, 2 moles of aluminum oxide are produced.|

III. For every 4 grams of aluminum that react with 3 grams of oxygen, 2 grams of aluminum oxide are produced.

React

Chapter 3 4

Which of the following statements are true concerning balanced chemical equations? There may be more than one true statement.

I. The number of molecules is conserved.

II. The coefficients tell you how much of each substance you have.

III.Atoms are neither created nor destroyed.

IV.The coefficients indicate the mass ratios of the substances used.

V.The sum of the coefficients on the reactant side equals the sum of the coefficients on the product side.

React

Chapter 3 5

The Atomic Mass ScaleThe Atomic Mass Scale1H weighs 1.6735 x 10-24 g and 16O 2.6560 x 10-23 g.We define: mass of 12C = exactly 12 amu.Using atomic mass units:

1 amu = 1.66054 x 10-24 g1 g = 6.02214 x 1023 amu

Atomic and Molecular WeightsAtomic and Molecular Weights

Chapter 3 6

Average Atomic MassAverage Atomic MassRelative atomic mass: average masses of isotopes:

Naturally occurring C: 98.892 % 12C + 1.108 % 13C.Average mass of C:

(0.98892)(12 amu) + (0.0108)(13 amu) = 12.011 amu.Atomic weight (AW) is also known as average atomic mass (atomic weight).Atomic weights are listed on the periodic table.

Atomic and Molecular WeightsAtomic and Molecular Weights

Chapter 3 7

Formula and Molecular WeightsFormula and Molecular WeightsFormula weights (FW): sum of AW for atoms in formula.

FW (H2SO4) = 2AW(H) + AW(S) + 4AW(O)= 2(1.0 amu) + (32.0 amu) + 4(16.0)

= 98.0 amuMolecular weight (MW) is the weight of the molecular formula.

MW(C6H12O6) = 6(12.0 amu) + 12(1.0 amu) + 6(16.0 amu)

Atomic and Molecular WeightsAtomic and Molecular Weights

Chapter 3 8

Percentage Composition from FormulasPercentage Composition from FormulasPercent composition is the atomic weight for each element divided by the formula weight of the compound multiplied by 100:

For iron in iron (III) oxide (Fe2O3)

Atomic and Molecular WeightsAtomic and Molecular Weights

massmass of element in compound

mass of compound% 100%

mass Fe%..

. 111 69159 69

100% 69 94%

Chapter 3 9

Percentage Composition from FormulasPercentage Composition from Formulas

Atomic and Molecular WeightsAtomic and Molecular Weights

Find the percent composition of carvone (C10H14O), an organic molecule responsible for the smell of spearmint oil.

massmass of element in compound

mass of compound% 100%

Chapter 3 10

Mole: convenient measure chemical quantities.1 mole of something = 6.022 x 1023 of that thing.Experimentally, 1 mole of 12C has a mass of exactly 12 g.

Molar MassMolar MassMolar mass: mass in grams of 1 mole of substance (units g/mol).

Mass of 1 mole of 12C = 12 g

ex: CO2 = 44.01 grams per mole

The MoleThe Mole

Chapter 3 11

Molar MassMolar MassMolar mass: sum of the molar masses of the atoms:

molar mass of N2 = 2 x (molar mass of N).Molar masses for elements are found on the periodic table.Formula weights are numerically equal to the molar mass.

The MoleThe Mole

Chapter 3 12

Interconverting Masses, Moles, and Numbers of Interconverting Masses, Moles, and Numbers of ParticlesParticles

The MoleThe Mole

Chapter 3 13

Interconverting Masses, Moles, and Numbers of Interconverting Masses, Moles, and Numbers of ParticlesParticles

A certain sample of calcium carbonate contains 4.86 moles. What is the mass of the sample? How many carbonate ions are present?

The MoleThe Mole

Chapter 3 14

Start with mass % of elements (i.e. empirical data) and calculate a formula, orStart with the formula and calculate the mass % elements.

Empirical Formulas from AnalysesEmpirical Formulas from Analyses

Chapter 3 15

Molecular Formula from Empirical FormulaMolecular Formula from Empirical FormulaDetermine the empirical and molecular formulas

of a compound that gives the following analysis (in mass percents): 71.65% Cl 24.27% C 4.07% H

The molar mass is known to be 98.96 g/mol.

Empirical Formulas from AnalysesEmpirical Formulas from Analyses

Chapter 3 16

Molecular Formula from Molecular Formula from Empirical FormulaEmpirical Formula

Caffeine contains 49.48% carbon, 5.15% hydrogen, 28.87% nitrogen, and 16.49% oxygen. The molar mass is 194.2 g/mol. Determine the molecular formula of caffeine.

Empirical Formulas from AnalysesEmpirical Formulas from Analyses

Chapter 3 17

Combustion AnalysisCombustion Analysis

Empirical formulas are determined by combustion analysis: This is a schematic diagram of the combustion device used to analyze substances for carbon and hydrogen. :

Empirical Formulas from AnalysesEmpirical Formulas from Analyses

Chapter 3 18

Writing a Chemical EquationWriting a Chemical Equation

Chemical symbols give a “before-and-after” picture of a chemical reaction

Reactants Products

MgO(s) + C(s) CO(g) + Mg (s)

magnesium oxide to form carbon monoxide

reacts with carbon and magnesium

Chapter 3 19

Balance Equations with Coefficients Balance Equations with Coefficients

Coefficients in front of formulas balance each type of atom

4NH3 + 5O2 4NO + 6H2O

4 N = 4 N

12 H = 12 H

10 O = 10 O

Symbols used in chemical reactions:

(g) (l) (s) (aq) e-

Chapter 3 20

The ratio of grams of reactant cannot be directly related to the grams of product.

Quantitative Information from Quantitative Information from Balanced EquationsBalanced Equations

Chapter 3 21

Quantitative Information from Quantitative Information from Balanced EquationsBalanced Equations

Solid lithium hydroxide is used in space vehicles to remove exhaled carbon dioxide from the living environment by forming solid lithium carbonate and liquid water. What mass of gaseous carbon dioxide can be absorbed by 1.00 kg of lithium hydroxide?

Chapter 3 22

If the reactants are not present in stoichiometric amounts, at end of reaction some reactants are still present (in excess).Limiting Reactant: one reactant that is consumed before the other reactant(s) thus limiting the amount of product

Limiting ReactantsLimiting Reactants

Chapter 3 23

Theoretical YieldsTheoretical YieldsThe amount of product predicted from stoichiometry taking into account limiting reagents is called the theoretical yield.The percent yield relates the actual yield (amount of material recovered in the laboratory) to the theoretical yield:

Limiting ReactantsLimiting Reactants

100yield lTheoretica

yield ActualYield %

Chapter 3 24

Theoretical YieldsTheoretical Yields

Methanol, CH3OH, is used as a fuel in race cars and is also a potential replacement for gasoline. It can be manufactured by combining carbon monoxide and hydrogen gases. Suppose 68.5 kg CO is reacted with 8.60 kg H2. Which reactant is limiting? What is the theoretical yield? If 3.57 x104 g is actually produced, what is the percent yield of methanol?

Limiting ReactantsLimiting Reactants

100yield lTheoretica

yield ActualYield %

Chapter 3 25

End of Chapter 3End of Chapter 3