StoichiometryStoichiometry

Molar Mass of CompoundsMolar Mass of Compounds

Molar Mass (MM)Molar Mass (MM) = = add up all the atomic add up all the atomic masses for the molecule (or compound)masses for the molecule (or compound)

Ex. Molar mass of CaClEx. Molar mass of CaCl22

Avg. Atomic mass of Calcium = 40.08gAvg. Atomic mass of Calcium = 40.08gAvg. Atomic mass of Chlorine = 35.45gAvg. Atomic mass of Chlorine = 35.45g

Molar Mass of calcium chloride = Molar Mass of calcium chloride = 40.08 g/mol Ca + (2 X 35.45) g/mol Cl40.08 g/mol Ca + (2 X 35.45) g/mol Cl

110.98 g/mol CaCl2

20

Ca  40.08

17Cl

35.45

Atoms or Molecules

Moles

Mass (grams)

Divide by 6.02 X 1023Multiply by 6.02 X 1023

Multiply by atomic/molar mass from periodic table

Divide by atomic/molar mass from periodic table

Volume (liters)

Divide 22.4 L @STP

Multiply by 22.4L@ STP

Other Stoichiometric CalculationsOther Stoichiometric CalculationsProblem-Solving ApproachProblem-Solving Approach

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

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 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!?!?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

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

Chemistry RecipesChemistry Recipes

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

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

Example: Example: 2 Na + Cl2 2 NaClThis reaction tells us that by mixing This reaction tells us that by mixing 22 moles of moles of

sodium with sodium with 1 1 mole of chlorine we will get mole of chlorine we will get 22 moles moles of sodium chlorideof sodium chloride

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

Mole -MoleMole -Mole 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

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

Volume-VolumeVolume-VolumeHow many liters of CHHow many liters of CH44 at STP are required to at STP are required to

completely react with 17.5 L of Ocompletely react with 17.5 L of O22??

CHCH44 + 2O + 2O22 CO CO22 + 2H + 2H22OO

17.5 L O2 2 L O2

1 L CH4 = 8.75 L CH4

Mole-Mass ConversionsMole-Mass Conversions Most of the time in chemistry, the amounts are Most of the time in chemistry, the amounts are

given in grams instead of molesgiven in grams instead of moles

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

EX: EX: How many grams of chlorine are required to How many grams of chlorine are required to react completely with 5.00 moles of sodium to react completely with 5.00 moles of sodium to produce sodium chloride?produce sodium 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

Mass-MoleMass-Mole We can start with mass and convert to moles of We can start with mass and convert to moles of

product or another reactantproduct or another reactant We use molar mass and the mole ratio to get to We use molar mass and the mole ratio to get to

moles of the compound of interestmoles of the compound of interestCalculate the number of moles of ethane Calculate the number of moles of ethane

(C(C22HH66) needed to produce 10.0 g of water) needed 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

Mass-Mass ConversionsMass-Mass Conversions Most often we are given a starting mass and want Most often we are given a starting mass and want

to find out the mass of a product we will get to find out the mass of a product we will get (called (called theoretical yield) or) or

how much of how much of another reactantanother reactant we need to we need to completely react with it (completely react with it (no leftover ingredients!)

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

Mass-Mass ConversionMass-Mass Conversion

Ex. Calculate how many grams of ammonia Ex. Calculate how many grams of ammonia are produced when you react are produced when you react 2.00g2.00g of of nitrogen with excess hydrogennitrogen 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

the bridge

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 Sometimes in chemistry we have more of Sometimes in chemistry we have more of

one reactant than we need to completely one reactant than we need to completely use up other reactant. use up other reactant.

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

The other reactant The other reactant limits how much productlimits how much product we get. Once it runs out, the reaction s. we 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, try To find the correct answer, try allall of the of the

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

The The lowerlower amount amount of 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 ALWAYS start with it!start with it!

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

Limiting reaction ExampleLimiting reaction Example

We get We get 49.4g49.4g of aluminum chlorideof aluminum chloride from the from the given amount of given amount of aluminumaluminum, but only , but only 43.9g43.9g of of aluminum chloride from the given amount of aluminum chloride from the given amount of chlorine.chlorine.

Therefore, Therefore, chlorinechlorine is the is the limiting reactant. . Once the 35.0g of chlorine is used up, the Once the 35.0g of chlorine is used up, the reaction comes to a complete .reaction 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 By calculating the amount of the excess reactant needed to completely excess reactant needed to completely react with the limiting reactant, subtract react with the limiting reactant, subtract that amount from the given amount to that amount from the given amount to find the amount of excessfind the amount of excess..

Can we find the amount of Can we find the amount of excess excess potassiumpotassium in the previous problem?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 You can recognize a limiting reactant problem because there is MORE THAN ONE GIVEN because there is MORE THAN ONE GIVEN AMOUNT.AMOUNT.

2.2. Convert ALL of the reactants to the SAME product Convert ALL of the reactants to the SAME product (pick any product you choose.)(pick 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 To find the amount of excess, subtract the amount

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

to start with the limiting reactant. You don’t have toto 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!

How do you get good at this?How do you get good at this?

Mass-Mass Problem:Mass-Mass Problem:

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

4Al + 3O2 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 1 x 2 x 101.96) ÷ (26.98 x 4 x 1) = 12.3 g Al2O3

are formed

Another example:Another example:If 10.1 g of Fe are added to a If 10.1 g of Fe are added to a

solution of Copper (II) Sulfate, how solution of Copper (II) Sulfate, how many grams of solid copper would many grams of solid copper would form?form?

2Fe + 3CuSO2Fe + 3CuSO44 Fe Fe22(SO(SO44))33 + 3Cu + 3Cu

Answer = 17.2 g Cu

Volume-Volume Calculations:Volume-Volume Calculations: How many liters of CHHow many liters of CH4 4 at STP are required to at STP are required to

completely react with 17.5 L of Ocompletely react with 17.5 L of O2 2 ??

CHCH44 + 2O + 2O22 CO CO22 + 2H + 2H22OO

17.5 L O2 2 mol O2

1 mol CH4 = 8.75 L CH4

Limiting Reagent & Percent YieldLimiting Reagent & Percent Yield

OBJECTIVES:OBJECTIVES:IdentifyIdentify the limiting reagent in a the limiting reagent in a

reaction.reaction.CalculateCalculate theoretical yield, percent theoretical yield, percent

yield, and the amount of excess yield, and the amount of excess reagent that remains unreacted given reagent that remains unreacted given appropriate informationappropriate information

““Limiting” ReagentLimiting” Reagent If you are given one dozen loaves of If you are given one dozen loaves of

bread, a gallon of mustard, and three bread, a gallon of mustard, and three pieces of salami, how many salami pieces of salami, how many salami sandwiches can you make?sandwiches can you make?

The The limiting reagentlimiting reagent is the reactant you is the reactant you run out of first.run out of first.

The The excess reagentexcess reagent is the one you have is the one you have left over.left over.

The limiting reagent determines how much The limiting reagent determines how much product you can makeproduct you can make

Limiting Reagents - CombustionLimiting Reagents - Combustion

How do you find out which is limited?How do you find out which is limited?

The chemical that makes the The chemical that makes the leastleast amount of productamount of product is the is the “limiting “limiting reagent”.reagent”.

You can recognize limiting reagent You can recognize limiting reagent problems because they will give you problems because they will give you 2 2 amounts of chemicalamounts of chemical

DoDo twotwo stoichiometry problems, one for stoichiometry problems, one for each reagent you are given.each reagent you are given.

If 10.6 g of copper reacts with If 10.6 g of copper reacts with 3.83 g sulfur, how many grams of the 3.83 g sulfur, how many grams of the product (copper (I) sulfide) will be formed?product (copper (I) sulfide) will be formed?

2Cu + S 2Cu + S Cu Cu22SS

10.6 g Cu 63.55g Cu 1 mol Cu

2 mol Cu 1 mol Cu2S

1 mol Cu2S

159.16 g Cu2S

= 13.3 g Cu2S

3.83 g S 32.06g S 1 mol S

1 mol S 1 mol Cu2S

1 mol Cu2S

159.16 g Cu2S

= 19.0 g Cu2S

Cu is the Limiting

Reagent, since it

produced less product.

Another example:Another example:

If 10.3 g of aluminum are If 10.3 g of aluminum are reacted with 51.7 g of CuSOreacted with 51.7 g of CuSO44 how much copper (grams) will how much copper (grams) will be produced?be produced?

2Al + 3CuSO2Al + 3CuSO44 → 3Cu + Al→ 3Cu + Al22(SO(SO44))33

the the CuSOCuSO44 is limited, so Cu = 20.6 g is limited, so Cu = 20.6 gHow much excess reagent will How much excess reagent will

remain? remain? Excess = 4.47 grams

The Concept of:

A little different type of yield than you had in Driver’s Education class.

What is Yield?What is Yield?

Yield is the amount of product made in a Yield is the amount of product made in a chemical reaction.chemical reaction.

There are three types:There are three types:1. 1. Actual yieldActual yield- what you actually get in the - what you actually get in the

lab when the chemicals are mixedlab when the chemicals are mixed2. 2. Theoretical yieldTheoretical yield- what the balanced - what the balanced

equation tells equation tells shouldshould be made be made3. 3. Percent yieldPercent yield = Actual = Actual

Theoretical Theoretical x 100

Example:Example:6.78 g of copper 6.78 g of copper is producedis produced when 3.92 g when 3.92 g

of Al are reacted with excess copper (II) of Al are reacted with excess copper (II) sulfate.sulfate.

2Al + 3 CuSO2Al + 3 CuSO44 Al Al22(SO(SO44))33 + 3Cu + 3Cu

What is the actual yield?What is the actual yield?

What is the theoretical yield?What is the theoretical yield?

What is the percent yield?What is the percent yield?

= 6.78 g Cu

= 13.8 g Cu

= 49.1 %

Details on YieldDetails on YieldPercent yield tells us how Percent yield tells us how

“efficient” a reaction is.“efficient” a reaction is.Percent yield Percent yield can notcan not be bigger be bigger

than 100 %.than 100 %.Theoretical yield will Theoretical yield will alwaysalways be be

larger than actual yield!larger than actual yield!Why?Why? Due to impure reactants; competing Due to impure reactants; competing

side reactions; loss of product in filtering or side reactions; loss of product in filtering or transferring between containers; measuringtransferring between containers; measuring

How do you get good at this?How do you get good at this?