Chapter 12

Introduction to Stoichiometry

Makin’ S’mores

Recipe

1 roasted marshmallow2 graham cracker squares1 chocolate

1 MM + 2 GC + 1 CH 1 s’more

How many S’mores can you make with the ingredients at hand?

Chocolate Chip Cookies!!1 cup butter 1/2 cup white sugar 1 cup packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups semisweet chocolate chipsMakes 3 dozen

How many eggs are needed to make 3 dozen cookies?

How much butter is needed for the amount of chocolate chips used?

How many eggs would we need to make 9 dozen cookies?

How much brown sugar would I need if I had 1 ½ cups white sugar?

Cookies and Chemistry…Huh!?!?

• Just like chocolate chip cookies have recipes, chemists have recipes as well

• Instead of calling them recipes, we call them reaction equations

• Furthermore, instead of using cups and teaspoons, we use moles

• Lastly, instead of eggs, butter, sugar, etc. we use chemical compounds as ingredients

Chemistry Recipes

• Looking at a reaction tells us how much of something you need to react with something else to get a product (like the cookie recipe)

• Be sure you have a balanced reaction before you start!

• Example: 2 Na + Cl2 2 NaCl• This reaction tells us that by mixing 2 moles of sodium

with 1 mole of chlorine we will get 2 moles of sodium chloride

• What if we wanted 4 moles of NaCl? 10 moles? 50 moles?

Flow Chart

Grams (given) Moles (given) Moles (Wanted) Grams (Wanted)

Divide by molar mass of given

Multiply by Molar Ratio

Multiply by molar mass of wanted

fWantedMolarMassoX

enMolesofGiv

tedMolesofWanX

fGivenMolarMasso

enGramsofGiv

2 HCl + Cu CuCl2 + H2

How many grams of hydrogen are produced when 67.12 g of hydrochloric acid are reacted with copper?

fWantedMolarMassoX

enMolesofGiv

tedMolesofWanX

fGivenMolarMasso

enGramsofGiv

22

22 86.11

016.2

2

1

/46.36

12.67gH

moleH

gHX

molesHCl

moleHX

molg

gHCl

Want

Given

2 NaClO3 2NaCl2 + 3O2

How many moles of sodium chloride are produced in the decomposition reaction of 145.73 grams of sodium chlorate?

fWantedMolarMassoX

enMolesofGiv

tedMolesofWanX

fGivenMolarMasso

enGramsofGiv

gNaClmolesNaClO

moleNaClX

molNaClOg

gNaClO37.1

2

2

/453.106

73.145

33

3

Want

Given

N2 + 3 H2 2NH3

How many moles of hydrogen are needed to completely react with two moles of nitrogen?

fWantedMolarMassoX

enMolesofGiv

tedMolesofWanX

fGivenMolarMasso

enGramsofGiv

22

22 61

32molesH

moleN

molesHX

molesN

Want

Given

The fermentation of glucose (C6H12O6) produces ethyl alcohol (C2H5OH) and CO2:

C6H12O6(aq) 2 C2H5OH(aq) + 2 CO2(g)

How many moles of CO2 are produced when 0.400 mol of C6H12O6 reacts in this fashion?

How many grams of C6H12O6 are needed to form 7.50 g of C2H5OH?

How many grams of CO2 form when 7.50 g of C2H5OH are produced?

STOICHIOMETRY!

Limiting Reactant: Cookies1 cup butter 1/2 cup white sugar 1 cup packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups semisweet chocolate chipsMakes 3 dozen

If we had the specified amount of all ingredients listed, could we make 4 dozen cookies?

What if we had 6 eggs and twice as much of everything else, could we make 9 dozen cookies?

What if we only had one egg, could we make 3 dozen cookies?

Limiting Reactant• Most of the time in chemistry we have more of one

reactant than we need to completely use up other reactant.

• That reactant is said to be in excessexcess (there is too much).

• The other reactant limits how much product we get. Once it runs out, the reaction s. This is called the limiting reactantlimiting reactant.

Aluminum hydroxide reacts with sulfuric acid as follows:

2 Al(OH)3(s) + 3 H2SO4(aq) Al2(SO4)3(aq) + 6 H2O(l)(78.002 g/mol) (98.0768 g/mol) (342.146 g/mol) (18.01 g/mol)

Which reagent is the limiting reactant when 35.10 g Al(OH)3 and 53.95 g H2SO4 are allowed to react? How many grams of Al2(SO4)3 can form under these conditions?

Limiting Reactant: Recap1. You can recognize a limiting reactant problem because there

is MORE THAN ONE GIVEN AMOUNT.2. Convert ALL of the reactants to the SAME product (pick any

product you choose.)3. The lowest answer is the correct answer.4. The reactant that gave you the lowest answer is the

LIMITING REACTANT.5. The other reactant(s) are in EXCESS.6. To find the amount of excess, subtract the amount used

from the given amount.7. If you have to find more than one product, be sure to start

with the limiting reactant. You don’t have to determine which is the LR over and over again!

YieldsThe amount of product calculated to form when all of the

limiting reagent reacts is called the theoretical yield.

The percent yield is related to the actual yield (what was actually recovered) and the theoretical yield (what could have been recovered in a perfect world).

% 100 yield ltheoretica

yield actual yieldPercent

Yields

What is the percent yield of the following reaction if the manufacturer obtains 21.4 g of CH3OH from 8.0 g of H2 in the presence of 25 g CO?

2 H2(g) + CO(g) CH3OH(l)

REMEMBER: Equations must be balancedTheoretical yield must be based on limiting reagent