# Stoichiometry

advertisement ```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 CaCl2
 Avg. Atomic mass of Calcium = 40.08g
 Avg. Atomic mass of Chlorine = 35.45g
 Molar 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 =
Ca3(PO4)2
Masses elements:
Ca = 40, P = 31, O = 16,
Molar Mass = 3x40 + 2x31 + 8x16
• = 310 g/mol
Calculations
molar mass
Grams
Avogadro’s number
Moles
particles
Everything must go through Moles!!!
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
2 eggs
What is the ratio of eggs to butter to make 3 dozen cookies? 1 cup butter
What is the ratio of butter to chocolate chips?
1 to 2
How many eggs would we need to make 9 dozen cookies?
How much brown sugar would I need if I had 1 &frac12; 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 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 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
 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?
50 mol H2 x 1 mol O2 = 25 mol O2
Mole ratio

2 mol H2
How much water would be produced?
50 mol H2 = 50 mol H2O (they have a 1:1 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 moles

Example: 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
Mole ratio
5.00 moles Na 1 mol Cl2
2 mol Na
70.90g Cl2
1 mol Cl2
Molar
mass
= 177g Cl2
Practice
Given Quantity  convert to moles  mole ratio  convert to
final units

Calculate the mass in grams of Iodine
required to react completely with 0.50
moles of aluminum.
2 Al + 3 I2  2 AlI3
0.50 moles Al X 3 moles I2
2 moles Al
X 253.8 g I2
moles
=190.35 g I2
Mass-Mole

We can also start with mass and convert to moles
of product or another reactant

Calculate the number of moles of ethane (C2H6)
needed to produce 10.0 g of water
2 C2H6 + 7 O2  4 CO2 + 6 H20
Given quantity
Convert
to moles
10.0 g H2O 1 mol H2O
Multiply by the
Mole ratio
2 mol C2H6 = 0.185
18.0 g H2O 6 mol H20
mol C2H6
Consider : 4NH3 + 5O2  6H2O + 4NO
How many grams of H2O are produced if 1.9 mol
of NH3 are combined with excess oxygen?
# g H2O=
1.9 mol NH3 x 6 mol H2O x 18.02 g H2O = 51.4 g
H 2O
4 mol NH3
1 mol H2O
How many grams of O2 are required to produce
0.3 mol of H2O?
# g O2=
0.3 mol H2O x
5 mol O2
32 g O2
8 g O2
x
=
6 mol H2O
1 mol O2
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
28.02g N2 1 mol N2
= 2.4 g NH3
17.06g NH3
1 mol 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
40 g
1 mole Ca3N2
3 mol Ca
x 148 g Ca3N2 = 2.47 g Ca3N2
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 = 0&ordm;C and 1 atmosphere pressure
Gases and Stoichiometry
2 H2O2 (l) ---&gt; 2 H2O (g) + O2 (g)
Decompose 1.1 g of H2O2 in a flask with a
volume of 2.50 L. What is the volume of O2
at STP?
Bombardier beetle
uses decomposition
of hydrogen peroxide
to defend itself.
19
Gases and Stoichiometry
2 H2O2 (l) ---&gt; 2 H2O (g) + O2 (g)
Decompose 1.1 g of H2O2 in a flask with a volume of
2.50 L. What is the volume of O2 at STP?
Solution
1.1 g H2O2
1 mol H2O2
34 g H2O2
1 mol O2
2 mol H2O2
22.4 L O2
1 mol O2
= 0.36 L O2 at STP
Molar
volume
20
21
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
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