Chapter 9 - Chapter 9 - StoichiometryStoichiometry

Sherlock Holmes, in Sir Arthur Conan Doyle’s A Study in Scarlet

“In solving a problem of this sort, the grand thing is to be able to reason backward. This is a very useful accomplishment, and a very easy one, but people do not practice it much.”

Chemical StoichiometryChemical Stoichiometry

StoichiometryStoichiometry - The study of - The study of quantitiesquantities of materials of materials consumedconsumed and and producedproduced in chemical in chemical reactions.reactions.

StoichiometStoichiometry ry

and theand the car car

ReviewReview: Atomic : Atomic MassesMasses

• Elements occur in nature as Elements occur in nature as mixtures of mixtures of isotopesisotopes

• Carbon =Carbon = 98.89% 98.89% 1212CC• 1.11% 1.11% 1313CC• <0.01% <0.01% 1414CC

• Carbon’s atomic mass = 12.01 Carbon’s atomic mass = 12.01 amuamu

ReviewReview: The Mole: The Mole

The number equal to the The number equal to the numbernumber of of carbon atoms in exactly 12 grams of carbon atoms in exactly 12 grams of pure pure 1212C.C.

1 mole1 mole of anything = of anything = 6.022 6.022 10 102323 units of that thingunits of that thing

The MoleThe Mole

Using Compound MassesUsing Compound Masses

ReviewReview: Molar : Molar MassMass

A substance’s A substance’s molar mass molar mass (molecular weight) is the mass in (molecular weight) is the mass in grams of one mole of the compound.grams of one mole of the compound.

COCO22 = 44.01 grams per mole = 44.01 grams per mole

HH22O = 18.02 grams per moleO = 18.02 grams per mole

Ca(OH)Ca(OH)22 = 74.10 grams per mole = 74.10 grams per mole

ReviewReview: Chemical : Chemical EquationsEquations

Chemical change involves a Chemical change involves a reorganization of reorganization of

the atoms in one or more substances.the atoms in one or more substances.CC22HH55OH + 3OOH + 3O22 2CO2CO22 + + 3H3H22OOreactantsreactants productsproducts

11 mole of ethanol mole of ethanol reacts with reacts with 33 moles of moles of oxygenoxygen

to produce to produce 22 moles of carbon dioxide moles of carbon dioxide and and 33 moles of watermoles of water

When the equation is balanced it has When the equation is balanced it has quantitative significance:quantitative significance:

Mole Mole RelationsRelations

Calculating Masses of Calculating Masses of Reactants and ProductsReactants and Products

1.1. Balance the equation.Balance the equation.

2.2. Convert mass to moles.Convert mass to moles.

3.3. Set up mole ratios.Set up mole ratios.

4.4. Use mole ratios to calculate moles Use mole ratios to calculate moles of desired substituent.of desired substituent.

5.5. Convert moles to grams, if Convert moles to grams, if necessary.necessary.

Working a Stoichiometry Working a Stoichiometry ProblemProblem

6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed.

1. Identify reactants and products and write the balanced equation.

Al + O2 Al2O3

b. What are the reactants?

a. Every reaction needs a yield sign!

c. What are the products?

d. What are the balanced coefficients?

4 3 2

Working a Stoichiometry Working a Stoichiometry ProblemProblem

6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed?

4 Al + 3 O2 2Al2O3

=6.50 g Al

? g Al2O3

1 mol Al

26.98 g Al 4 mol Al

2 mol Al2O3

1 mol Al2O3

101.96 g Al2O3

6.50 x 2 x 101.96 ÷ 26.98 ÷ 4 =

12.3 g Al2O3

Limiting Limiting ReactantReactant

The The limiting reactantlimiting reactant is the is the reactant reactant

that is that is consumed firstconsumed first,, limiting the amounts of products limiting the amounts of products formed.formed.

Limiting Reagents - Limiting Reagents - CombustionCombustion

Solving a Stoichiometry Solving a Stoichiometry ProblemProblem

1.1. Balance the equation.Balance the equation.

2.2. Convert masses to moles.Convert masses to moles.

3.3. Determine which reactant is limiting.Determine which reactant is limiting.

4.4. Use moles of limiting reactant and Use moles of limiting reactant and mole ratios to find moles of desired mole ratios to find moles of desired product.product.

5.5. Convert from moles to grams.Convert from moles to grams.