Stoichiometry

Review of Molar Mass

The molar mass (MM) of a compound is all the atomic masses for the molecule (or compound) added up.Ex. Molar mass of CaCl2Avg. Atomic mass of Calcium = 40.08gAvg. Atomic mass of Chlorine = 35.45gMolar Mass of calcium chloride =

40.08 g/mol Ca + (2 X 35.45) g/mol Cl 110.98 g/mol CaCl2

20

Ca  40.08 17

Cl 35.45

Practice

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

Ca = 40, P = 31, O = 16, Molar Mass = 3x40 + 2x31 + 8x16

• = 310 g/mol

Ca3(PO4)2

molar mass Avogadro’s number Grams Moles particles

Everything must go through Moles!!!

Calculations

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

What is the ratio of eggs to butter to make 3 dozen cookies?

What is the ratio of butter to chocolate chips?

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?

2 eggs

1 cup butter

1 to 2

Cookies and Chemistry…Huh!?!?

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

Instead of calling them recipes, we call them chemical 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

Be sure you have a balanced reaction before you start!Example: 2 Na + Cl2 2 NaClThis reaction tells us that by mixing 2 moles of

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

How much Na and Cl2 would be need if we wanted to make 4 moles of NaCl?

4 moles of Na and 2 moles of Cl2

Practice Write the balanced reaction for hydrogen gas reacting

with oxygen gas. 2 H2 + O2 2 H2O

What if we had 50 moles of hydrogen, how much oxygen would react with it?

How much water would be produced?

50 mol H2 x 1 mol O2 = 25 mol O2

2 mol H2

50 mol H2 = 50 mol H2O (they have a 1:1 ratio)

Mole ratio

Mole Ratios

The mole ratio is used to switch between substances in a chemical equation.

Example: How many moles of chlorine are needed to react with 5 moles of sodium?

2 Na + Cl2 2 NaCl

5 moles Na 1 mol Cl2

2 mol Na= 2.5 moles Cl2

Mole-Mole Conversions

How many moles of sodium chloride will be produced if 2.6 moles of chlorine gas reacts with excess sodium metal?

2 Na + Cl2 2 NaCl

2.6 moles Cl2 x 2 mol NaCl = 5.20 mol of NaCl 1 mol Cl2

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

in grams instead of molesExample: How many grams of chlorine are

required to react completely with 5.00 moles of sodium to produce sodium chloride?

2 Na + Cl2 2 NaCl

5.00 moles Na 1 mol Cl2 70.90g Cl2

2 mol Na 1 mol Cl2

= 177g Cl2

Mole ratio

Molar mass

Practice

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

2 Al + 3 I2 2 AlI3

Given Quantity convert to moles mole ratio convert to final units

=190.35 g I2

0.50 moles Al X 3 moles I2

2 moles Al

X 253.8 g I2

moles

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

of product or another reactantCalculate the number of moles of ethane (C2H6)

needed to produce 10.0 g of water

2 C2H6 + 7 O2 4 CO2 + 6 H20

10.0 g H2O 1 mol H2O 2 mol C2H6

18.0 g H2O 6 mol H20

= 0.185 mol C2H6

Given quantity Convert to moles

Multiply by the Mole ratio

Consider : 4NH3 + 5O2 6H2O + 4NOHow many grams of H2O are produced if 1.9 mol

of NH3 are combined with excess oxygen?

How many grams of O2 are required to produce 0.3 mol of H2O?

6 mol H2O

4 mol NH3

x

# g H2O=

1.9 mol NH3 51.4 g H2O

= 18.02 g H2O

1 mol H2Ox

5 mol O2

6 mol H2Ox

# g O2=

0.3 mol H2O8 g O2= 32 g O2

1 mol O2

x

Mass-Mass Conversions

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

Given Quantity convert to moles mole ratio convert to final units

Mass-Mass Conversion

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

N2 + 3 H2 2 NH3

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

Practice: Grams to Grams

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

3 Ca + N2 Ca3N2

2.00 g Ca x 1 mole Ca x 1 mole Ca3N2 x 148 g Ca3N2 = 2.47 g Ca3N2

40 g 3 mol Ca 1 mole

Given amount change to moles multiply by the mole ratio change to (put the one on top that you grams are solving for)

Gases and Stoiochiometry

Avogadro's Hypothesis - at the same temperature and pressure equal volumes of gas have the same number of particles.

At STP 1 mole of any gas has a volume of 22.4 L

STP = 0ºC and 1 atmosphere pressure

1919Gases and Gases and StoichiometryStoichiometry

2 H2 H22OO2 2 (l) ---> 2 H(l) ---> 2 H22O (g) + OO (g) + O2 2 (g)(g)

Decompose 1.1 g of HDecompose 1.1 g of H22OO22 in a flask with a in a flask with a volume of 2.50 L. What is the volume of Ovolume of 2.50 L. What is the volume of O22 at STP?at STP?

Bombardier beetle Bombardier beetle uses decomposition uses decomposition of hydrogen peroxide of hydrogen peroxide to defend itself.to defend itself.

2020Gases and Gases and StoichiometryStoichiometry

2 H2 H22OO2 2 (l) ---> 2 H(l) ---> 2 H22O (g) + OO (g) + O2 2 (g)(g)

Decompose 1.1 g of HDecompose 1.1 g of H22OO22 in a flask with a volume of in a flask with a volume of 2.50 L. What is the volume of O2.50 L. What is the volume of O22 at STP? at STP?

SolutionSolution

1.1 g1.1 g HH22OO22 1 mol H 1 mol H22OO22 1 mol O 1 mol O22 22.4 L O 22.4 L O22

34 g H34 g H22OO22 2 mol H 2 mol H22OO22 1 mol O 1 mol O22

= 0.36 L O2 at STP

Molar volume

2121

Gas Stoichiometry: Practice!

A. What is the volume at STP of 4.00 g of CH4?

4.00 g CH4 x 1 mole x 22.4 L = 5.6 L of CH4

16 g 1 mole

B. How many grams of CO2 are produced from the

combustion of 20 grams CH4 at STP?

CH4 + 2 O2 CO2 + 2 H2O

20 g CH4 x 1 mole x 1 mole CO2 x 22.4 L = 28 L CO2

16 g 1 mole CH4 1 mole