StoichiometryStoichiometry

The Mole: ReviewThe Mole: Review

A counting unitA counting unit Similar to a dozen, except instead Similar to a dozen, except instead

of 12, itof 12, it’’s s 602,000,000,000,000,000,000,000602,000,000,000,000,000,000,000

6.02 X 106.02 X 1023 23 (in scientific notation)(in scientific notation) This number is named in honor of This number is named in honor of

Amedeo Avogadro (1776 – 1856)Amedeo Avogadro (1776 – 1856)

Review of Molar Mass of Review of Molar Mass of CompoundsCompounds

The molar mass (MM) of a compound is The molar mass (MM) of a compound is determined the same way, except now you add determined the same way, except now you add up all the atomic masses for the molecule (or up all the atomic masses for the molecule (or compound)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 = 35.45gMolar 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 CaCl 110.98 g/mol CaCl22

20

Ca  40.08

17Cl

35.45

The Mole Map

Mole

Mass

Gas Volume @ STP

# Particles

x

x

x

22.4L6.02 x 10

23

Mol

ar

Mas

s

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

Example: 2 Na + ClExample: 2 Na + Cl2 2 2 NaCl 2 NaClThis reaction tells us that by mixing 2 moles This reaction tells us that by mixing 2 moles

of sodium with 1 mole of chlorine we will get of sodium with 1 mole of chlorine we will get 2 moles of sodium chloride2 moles of sodium chloride

What if we wanted 4 moles of NaCl? 10 What if we wanted 4 moles of NaCl? 10 moles? moles? 50 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 are needed?How many moles of reactants are needed? What if we wanted 4 moles of water?What if we wanted 4 moles of water? What if we had 3 moles of oxygen, how much hydrogen What if we had 3 moles of oxygen, how much hydrogen

would we need to react and how much water would we would we need to react and how much water would we get?get?

What if we had 50 moles of hydrogen, how much oxygen What if we had 50 moles of hydrogen, how much oxygen would we need and how much water produced? would we need and how much water produced?

2 mol H2 1 mol O2

4 mol H2

2 mol O2

6 mol H2, 6 mol H2O

25 mol O2, 50 mol H2O

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 chloride will How many moles of sodium chloride will

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

2 Na + Cl2 Na + Cl22 2 NaCl 2 NaCl

2.6 moles Na 1 mol Cl2

2 mol Na= 1.3 moles Cl2

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

Example: How many grams of chlorine are required Example: How many grams of chlorine are required to react completely with 5.00 moles of sodium to 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

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.

2 Al + 3 I2 Al + 3 I2 2 2 AlI 2 AlI3 3

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

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 oxide4 Al + 3 O4 Al + 3 O2 2 2 Al 2 Al22OO33

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

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?

3 Ca + N3 Ca + N22 Ca Ca33NN22

The Cheese Sandwich AnalogyThe Cheese Sandwich Analogy

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 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 each each 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 product! You can’’t compare to 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.

15.0 g K 1 mol K 2 mol KI 166.00 g KI

39.10g K 2 mol K 1 mol KI= 63.7 g KI

15.0 g I2 1 mol K 2 mol KI 166.00 g KI

253.00g I2 1 mol I2 1 mol KI= 19.6g KI

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!

More PracticeMore Practice

Consider this chemical reaction:Consider this chemical reaction:

CaCOCaCO33 + HCl + HCl CaCl CaCl22 + H + H22O + COO + CO22

(This is what happens when you take an (This is what happens when you take an antacid)antacid)

Balance this equation.Balance this equation.How much calcium carbonate would you How much calcium carbonate would you

need to need to ““use upuse up”” 3.6 g of hydrogen 3.6 g of hydrogen chloride?chloride?

You have a few minutes to work this out…You have a few minutes to work this out…

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