StoichiometryStoichiometryChemistry I: Chapter 12Chemistry I: Chapter 12

Chemistry IH: Chapter 12Chemistry IH: Chapter 12

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Stoichiometry

The method of measuring amounts of

substances and relating them to each

other.

MoleA unit of measurement that is

equal to 6.02 x 1023

Also known as Avogadro’s constant (number)

It is the number of atoms of an element in the atomic mass (in grams) of that element.

Molar MassThe molar mass (MM) of an element

is how much a mole of atoms of that element weighs. Equal to the atomic mass in grams.

Ex: what’s the molar mass of BeEx: What is the molar mass of O2?

Cont…

The molar mass (MM) of a compound is how much a mole of formula units of that element weighs. Equal to the molecular mass in grams.

Reminder: A formula unit is the smallest ratio of atoms in a unit of a compound. It is designated by its formula. Ex: NaCl, H2O

Ex: what is the molar mass of NaCl? Ex: What is the molar mass of H2O?

PracticePractice

Calculate the Molar Mass of Calculate the Molar Mass of calcium phosphatecalcium phosphate Formula = Formula = Masses elements:Masses elements:

Molar Mass = Molar Mass =

Ca3(PO4)2

ConversionsConversionsWe use We use

conversions all conversions all the time!the time!

Ex: What is Ex: What is another fraction another fraction we can use to we can use to express ½? express ½?

To convert, we To convert, we multiply times a multiply times a conversion conversion factor, which is factor, which is equal to 1.equal to 1.

½ x 5/5 = 5/10½ x 5/5 = 5/10½ = 2/4 = 3/6 = 5/10

FlowchartFlowchartAtoms or Molecules

Moles

Mass (grams)

Divide by 6.02 X 1023

Multiply by 6.02 X 1023 Multiply by atomic/molar mass from periodic table

Divide by atomic/molar mass from periodic table

molar mass Avogadro’s numbermolar mass Avogadro’s number Grams Grams MolesMoles particles particles

Everything must go through Everything must go through Moles!!!Moles!!!

CalculationsCalculations

Chocolate Chip Cookies!!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 chips

Makes 3 dozenHow 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!?!?Cookies and Chemistry…Huh!?!?

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

Instead of calling them Instead of calling them recipes, we call them reaction recipes, we call them reaction equationsequations

Furthermore, instead of using Furthermore, instead of using cups and teaspoons, we use cups and teaspoons, we use molesmoles

Last, instead of eggs, butter, Last, instead of eggs, butter, sugar, etc. we use chemical sugar, etc. we use chemical compounds as ingredientscompounds as ingredients

Chemistry RecipesChemistry RecipesLooking at a reaction tells Looking at a reaction tells

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

Chemistry RecipesChemistry RecipesBe sure you have a balanced Be sure you have a balanced

reaction before you startreaction before you startExample: 2 Na + ClExample: 2 Na + Cl2 2 2 NaCl 2 NaClThis reaction tells us that by mixing This reaction tells us that by mixing

2 moles of Na with 1 mole of Cl we 2 moles of Na with 1 mole of Cl we will get 2 moles of sodium chloridewill get 2 moles of sodium chloride

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

PracticePractice

Write the balanced reaction for hydrogen gas Write the balanced reaction for hydrogen gas reacting with oxygen gas.reacting with oxygen gas.

2 H2 H22 + O + O22 2 H 2 H22OO

How many moles of reactants needed?How many moles of reactants needed?

What if we wanted 4 moles of water?What if we wanted 4 moles of water?

What if we had 3 moles of OWhat if we had 3 moles of O22 , how , how

much Hmuch H22 would we need to react and would we need to react and

how much water would we get?how much water would we get?

What if we had 50 moles of HWhat if we had 50 moles of H22? ?

Mole RatiosMole Ratios

These mole ratios can be used to calculate These mole ratios can be used to calculate the moles of one chemical from the given the moles of one chemical from the given amount of a different chemical amount of a different chemical

Example: How many moles of chlorine is Example: How many moles of chlorine is needed to react with 5 moles of sodium needed to react with 5 moles of sodium (without any sodium left over)?(without any sodium left over)?

2 Na + Cl2 Na + Cl22 2 NaCl 2 NaCl

5 moles Na 1 mol Cl2

2 mol Na= 2.5 moles Cl2

Mole-Mole ConversionsMole-Mole ConversionsHow many moles of sodium How many moles of sodium

chloride will be produced if chloride will be produced if you react 2.6 moles of you react 2.6 moles of chlorine gas with an excess chlorine gas with an excess (more than you need) of (more than you need) of sodium metal?sodium metal?

Mole-Mass ConversionsMole-Mass ConversionsExample: How many grams of Example: How many grams of chlorine are required to react chlorine are required to react completely with 5.00 moles of completely with 5.00 moles of sodium to produce sodium sodium to produce sodium chloride?chloride?

2 Na + Cl2 Na + Cl22 2 NaCl 2 NaCl

5.00 moles Na 1 mol Cl2 70.90g Cl2

2 mol Na 1 mol Cl2

= 177g Cl2

Mole-Mass ConversionsMole-Mass ConversionsMost of the time in chemistry, Most of the time in chemistry,

the amounts are given in grams the amounts are given in grams instead of molesinstead of moles

We still go through moles and We still go through moles and use the mole ratio, but now we use the mole ratio, but now we also use molar mass to get to also use molar mass to get to gramsgrams

Mass-MoleMass-MoleWe can also start with mass and convert to We can also start with mass and convert to

moles of product or another reactantmoles of product or another reactantWe use molar mass and the mole ratio to get We use molar mass and the mole ratio to get

to moles of the compound of interestto moles of the compound of interestCalculate the number of moles of ethane (CCalculate the number of moles of ethane (C22HH66) )

needed to produce 10.0 g of waterneeded to produce 10.0 g of water 2 C2 C22HH66 + 7 O + 7 O22 4 CO 4 CO22 + 6 H + 6 H220 0

10.0 g H2O 1 mol H2O 2 mol C2H6

18.0 g H2O 6 mol H20

= 0.185 mol C2H6

PracticePractice

Calculate the mass in grams of Iodine Calculate the mass in grams of Iodine required to react completely with 0.50 required to react completely with 0.50 moles of aluminum.moles of aluminum.

PracticePracticeCalculate how many moles of oxygen are Calculate how many moles of oxygen are

required to make 10.0 g of aluminum oxiderequired to make 10.0 g of aluminum oxide

Mass-Mass ConversionsMass-Mass Conversions

Most often we are given a starting mass Most often we are given a starting mass and want to find out the mass of a product and want to find out the mass of a product we will get (called theoretical yield) or how we will get (called theoretical yield) or how much of another reactant we need to much of another reactant we need to completely react with it (no leftover completely react with it (no leftover ingredients!)ingredients!)

Now we must go from grams to moles, Now we must go from grams to moles, mole ratio, and back to grams of mole ratio, and back to grams of compound we are interested incompound we are interested in

Mass-Mass ConversionMass-Mass Conversion

Ex. Calculate how many grams of Ex. Calculate how many grams of ammonia are produced when you react ammonia are produced when you react 2.00g of nitrogen with excess hydrogen.2.00g of nitrogen with excess hydrogen.

NN2 2 + 3 H+ 3 H2 2 2 NH 2 NH33

2.00g N2 1 mol N2 2 mol NH3 17.06g NH3

28.02g N2 1 mol N2 1 mol NH3

= 2.4 g NH3

PracticePractice

How many grams of calcium nitride are How many grams of calcium nitride are produced when 2.00 g of calcium reacts produced when 2.00 g of calcium reacts with an excess of nitrogen?with an excess of nitrogen?

Limiting Reactant: CookiesLimiting 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 chips

Makes 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 ReactantLimiting Reactant

Most of the time in chemistry we have more of Most of the time in chemistry we have more of one reactant than we need to completely use one reactant than we need to completely use up other reactant.up other reactant.

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

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

Limiting ReactantLimiting Reactant To find the correct answer, we have to try To find the correct answer, we have to try allall of of

the reactants. We have to calculate how much the reactants. We have to calculate how much of of aa product we can get from product we can get from eacheach of the of the reactants to determine which reactant is the reactants to determine which reactant is the limiting one.limiting one.

The The lowerlower amount of amount of aa product is the correct product is the correct answer.answer.

The reactant that makes the least amount of The reactant that makes the least amount of product is the product is the limiting reactantlimiting reactant. Once you . Once you determine the limiting reactant, you should determine the limiting reactant, you should ALWAYS start with it!ALWAYS start with it!

Be sure to pick Be sure to pick aa product! You can’t compare to product! You can’t compare to see which is greater and which is lower unless see which is greater and which is lower unless the product is the same!the product is the same!

Limiting Reactant: ExampleLimiting Reactant: Example 10.0g of aluminum reacts with 35.0 grams of 10.0g of aluminum reacts with 35.0 grams of

chlorine gas to produce aluminum chloride. Which chlorine gas to produce aluminum chloride. Which reactant is limiting, which is in excess, and how reactant is limiting, which is in excess, and how much product is produced?much product is produced?

2 Al + 3 Cl2 Al + 3 Cl22 2 AlCl 2 AlCl33 Start with Al:Start with Al:

Now ClNow Cl22::

10.0 g Al 1 mol Al 2 mol AlCl3 133.5 g AlCl3

27.0 g Al 2 mol Al 1 mol AlCl3

= 49.4g AlCl3

35.0g Cl2 1 mol Cl2 2 mol AlCl3 133.5 g AlCl3

71.0 g Cl2 3 mol Cl2 1 mol AlCl3

= 43.9g AlCl3

LimitingLimitingReactantReactant

LR Example ContinuedLR Example Continued

We get We get 49.4g49.4g of aluminum chloride from the given of aluminum chloride from the given amount of aluminum, but only amount of aluminum, but only 43.9g43.9g of aluminum of aluminum chloride from the given amount of chlorine. chloride from the given amount of chlorine. Therefore, chlorine is the limiting reactant. Once Therefore, chlorine is the limiting reactant. Once the 35.0g of chlorine is used up, the reaction the 35.0g of chlorine is used up, the reaction comes to a complete .comes to a complete .

Limiting Reactant PracticeLimiting Reactant Practice

15.0 g of potassium reacts with 15.0 g of 15.0 g of potassium reacts with 15.0 g of iodine. Calculate which reactant is limiting iodine. Calculate which reactant is limiting and how much product is made.and how much product is made.

Finding the Amount of ExcessFinding the Amount of Excess

By calculating the amount of the excess By calculating the amount of the excess reactant needed to completely react with reactant needed to completely react with the limiting reactant, we can subtract that the limiting reactant, we can subtract that amount from the given amount to find the amount from the given amount to find the amount of excess.amount of excess.

Can we find the amount of excess Can we find the amount of excess potassium in the previous problem?potassium in the previous problem?

Finding Excess PracticeFinding Excess Practice 15.0 g of potassium reacts with 15.0 g of iodine. 15.0 g of potassium reacts with 15.0 g of iodine.

2 K + I2 K + I22 2 KI 2 KI We found that Iodine is the limiting reactant, and We found that Iodine is the limiting reactant, and

19.6 g of potassium iodide are produced.19.6 g of potassium iodide are produced.

15.0 g I2 1 mol I2 2 mol K 39.1 g K

254 g I2 1 mol I2 1 mol K= 4.62 g K USED!

15.0 g K – 4.62 g K = 10.38 g K EXCESS

Given amount of excess reactant

Amount of excess reactant actually used

Note that we started with the limiting reactant! Once you determine the LR, you should only start with it!

Limiting Reactant: RecapLimiting Reactant: Recap

1.1. You can recognize a limiting reactant problem because You can recognize a limiting reactant problem because there is MORE THAN ONE GIVEN AMOUNT.there is MORE THAN ONE GIVEN AMOUNT.

2.2. Convert ALL of the reactants to the SAME product (pick Convert ALL of the reactants to the SAME product (pick any product you choose.)any product you choose.)

3.3. The lowest answer is the correct answer.The lowest answer is the correct answer.4.4. The reactant that gave you the lowest answer is the The reactant that gave you the lowest answer is the

LIMITING REACTANT.LIMITING REACTANT.5.5. The other reactant(s) are in EXCESS.The other reactant(s) are in EXCESS.6.6. To find the amount of excess, subtract the amount used To find the amount of excess, subtract the amount used

from the given amount.from the given amount.7.7. If you have to find more than one product, be sure to If you have to find more than one product, be sure to

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