Reaction stoichiometry involves the mass relationships between reactants and products in a chemical reaction.
It is based on chemical equations similar to the ones studied in the last section.
All reaction stoichiometry calculations start with a balanced equation.
C O
You will need to be familiar with gram/mole relationships as studied earlier this year.

The mole enables chemists to move from the microscopic world of atoms and molecules to the real world of grams .
Stoichiometry problems are classified between the information given in the problem and the information you are expected to find, the unknown.
The given and the unknown may be expressed in grams or moles.
The masses in the reaction are usually expressed in grams.
Definition of mole: mole of a substance = grams of substance/MW of substance

i. molar ratios in a balanced equation.
ii. molar masses of reactants and products.
iii. balancing equations.
iv. conversions between grams and moles.

Reaction between magnesium and oxygen to form magnesium oxide. ( fireworks)
2 MgO(s) (balanced) 2 Mg(s) + O
2
(g)
Mole Ratios:
2 : 1 : 2

Stoichiometry (working with ratios)
Ratios are found within a chemical equation.
2 1
2

2 H
2
1
2 coefficients give MOLAR RATIOS
2 moles of HCl react with 1 mole of Ba(OH)
2 and 1 mole of BaCl
2 to form 2 moles of H
2
O

1) N
2
+ 3 H
2
---> 2 NH
3
Write the mole ratios for N
2 to H
2
.
to H
2 and NH
3

A substance’s molar mass (molecular weight) is the mass in grams of one mole of the compound.
CO
2
= 44.01 grams per mole
H
2
O = 18.02 grams per mole
Ca(OH)
2
= 74.10 grams per mole

C
2
H
5
OH + 3O
2 reactants
 2CO
2
+ 3H
2
O products
1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water

Problem Type 1:
When you are given the amount of starting material in a reaction in moles and asked to calculate the amount of product in moles: amount of reactant in moles amount of product in moles
Problem Type 2:
When you are given the amount of starting material in moles and asked to calculate the mass of product in grams: amount of reactant in moles amount of product in moles mass of product in grams

Problem Type 3:
When you are given the mass of starting material in grams and asked to calculate the amount of product in moles. amount of reactant in grams amount of reactant in moles amount of product in moles
Problem Type 4:
1.
When you are given the mass of starting material in grams and asked to calculate the amount of product in grams. amount of reactant amount of reactant in moles amount of product in moles amount of product in grams in grams

Problem Type 1: Given and unknown quantities are in moles
Amount in moles of known substance
Amount in moles of unknown substance
How many moles of lithium hydroxide are required to react with 20 moles of CO 2
C L
H O

Given: amount of CO 2 = 20 moles
Unknown: amount of LiOH in moles
Amount of CO 2 in moles Amount of LiOH in moles mol CO 2 x mol LiOH / mol CO 2 = mol LiOH
20 mol CO 2 x 2 mol LiOH / 1 mol CO 2 = 40 mol LiOH mole ratio

Problem Type 1
Mole – Mole Conversions
When N
2
O
5 is heated, it decomposes:
2 N
2
O
5
(g)

4 NO
2
(g) + O
2
(g) a. How many moles of NO
2 can be produced from 4.3 moles of N
2
O
5
?
2 N
2
O
5
(g)

4 NO
2
(g) + O
2
(g)
4.3 mol ? mol Units match

Mole – Mole Conversions
When N
2
O
5 is heated, it decomposes:
2 N
2
O
5
(g)

4 NO
2
(g) + O
2
(g) a. How many moles of NO
2
2 N
2
O
5
(g) can be produced from 4.3 moles of N

4 NO
2
(g) + O
2
(g)
2
O
5
?
4.3 mol ? mol
Units match
4.3 mol N
2
O
5
4 mol NO
2
2 mol N
2
O
5
8.6
2

b. How many moles of O
2 can be produced from 4.3 moles of N
2
O
5
?
2 N
2
O
5
(g)

4 NO
2
(g) + O
2
(g)
4.3 mol ? mol
4.3 mol N
2
O
5
1 mol O
2
2mol N
2
O
5
2.2
2

Problem Type 1: Given and unknown quantities are in moles
Amount in moles of known substance
Amount in moles of unknown substance
How many moles of ammonia are produced when 6 moles of hydrogen gas react with an excess of nitrogen gas.
H
2
+ N
2
N H
3
u n b a la n c e d e q u a t io n
3 H
2
+ N
2
2 N H
3
b a la n c e d e q u a t io n

Given: amount of H
2
= 6 moles
Unknown: amount of NH
3 in moles
Amount of H
2 in moles Amount of NH
3 in moles
(mol H
2
) x (mol NH
3
/ mol H
2
) = mol NH
3
(6 mol H
2
) x (2 mol NH
3
/ 3 mol H
2
) = 4 mol NH
3
Mole ratio

Problem Type 2: Given amount is in moles and unknown quantity is in grams
Amount in moles of known substance
Amount in grams of unknown substance
Problem Type 3: Given amount is in grams and unknown quantities are in moles
Amount in grams of known substance
Amount in moles of unknown substance

Problem Type 2 mole ↔ gram
In plants when carbon dioxide reacts with water it produces glucose and oxygen:
6 CO
2
+ 6 H
2
O(l)

C
6
H
12
O
6
(s) + 6 O
2
(g)
How many grams of C
6
H
12
O
6 is produced when 3.0 mol of water react with carbon dioxide?
6 CO
2
+ 6 H
2
O

C
6
H
12
O
6
+ 6O
2
3.0 mol ? grams
Units match
3.0 mol H
2
O 1 mol C
6
H
12
O
6
6 mol H
2
O
180 .
2 g C
6
H
12
O
6
1 mol C
6
H
12
O
6
90
6
H
12
O
6

Problem Type 2 mole ↔ gram
In plants when carbon dioxide reacts with water it produces glucose and oxygen:
6 CO
2
+ 6 H
2
O(l)

C
6
H
12
O
6
(s) + 6 O
2
(g)
How many grams of CO
2 is needed to react with 3.0 mol of water?
6 CO
2
? grams
+ 6 H
2
O

C
6
H
12
O
6
3.0 mol
+ 6O
2
Units match
3.0 mol H
2
O 6 mol CO
2
6 mol H
2
O
4 4g CO
2
1 mol CO
2
132
2

Problem Type 2 mole ↔ gram
When magnesium burns in air, it combines with oxygen to form magnesium oxide according to the following equation:
2 Mg + O
2
(g)

2 Mg O(s)
How many grams of MgO is produced from 2.0 mol of magnesium?
2 Mg + O
2
2.0 mol

2 MgO
? grams
Units match
2.0 mol Mg
2 mol MgO
2 mol Mg
4 0g MgO
1 mol MgO

Problem Type 3 gram ↔ mole
When N
2
O
5
2 N
2
O is heated, it decomposes:
5
(g)

4 NO
2
(g) + O
2
(g) a. How many moles of N
2
O
5 were used if 210g of NO
2
2 N
2
O
5
(g)

4 NO
2
(g) + O
2
(g) were produced?
? moles 210g
Units match
210 g NO
2 mol NO
2
46 .
0 g NO
2
2
4 mol mol
N
2
O
5
NO
2
2.28
2
O
5

Problem Type 3 gram ↔ mole
Nitric acid is produced from the catalytic oxidation of ammonia
NH
3
(g) + O
2
(g)

NO(g) + H
2
O(g) a. How many moles of NO were produced from 824g of NH
3
?
4 NH
3
(g) + 5 O
2
824g

4 NO(g) + 6 H
2
O(g)
? moles
Units match
824 g NH
3
1 mol NH
3
17 .
04 g NH
3
4 mol NO
4 mol NH
3

• Balance the chemical equation correctly
• Using the molar mass of the given substance, convert the mass given to moles.
• Determine the molar ratio.
• Using the molar mass of the unknown substance, convert the moles just calculated to mass.
Amount in grams of known substance mw
Amount in moles of known substance molar ratio
Amount in moles of unknown substance mw
Amount in grams of unknown substance

Problem Type 4 grams ↔ grams
When N
2
O
5 is heated, it decomposes:
How many grams of N
2
O
5 are needed to produce 75.0 grams of O
2
?
2 N
2
O
5
(g)

4 NO
2
(g) + O
2
(g)
? grams 75.0 g
75.0
g O
2 mol O
2
32.0
g O
2
2 mol N
2
O
5
1mol O
2
108 g N
2
O
5 mol N
2
O
5
= grams N
2
O
5

6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed.
1. Identify reactants and products and write the balanced equation.
Al + O
2
Al
2
O
3 a. What are the reactants?
b. What are the products?
c. What are the balanced coefficients?

6.50 grams of aluminum reacts with an excess of oxygen.
How many grams of aluminum oxide are formed?
4 Al + 3 O
2
 2Al
2
O
3
6.50 g Al 1 mol Al 2 mol Al
2
O
3
26.98 g Al 4 mol Al
101.96 g Al
2
O
3
1 mol Al
2
O
3
=
6.50 x 2 x 101.96
÷ 26.98 ÷ 4 = 12.3 g Al
2
O
3
? g Al
2
O
3

Acetylene gas (C
2
H
2
) is produced by adding water to calcium carbide:
How many grams of acetylene are produced by adding water to
5.0 grams of CaC
2
?
CaC
2
+ 2H
2
O

C
2
H
2
+ Ca(OH)
2
Balanced?
5.0 g ? grams
5.0
g CaC
2
1 mol CaC
2
64.1
g CaC
2
1 mol C
2
H
2
1mol CaC
2
26g C
2
H
2
1 mol C
2
H
2
= grams C
2
H
2

How many moles of CaC
2 with 49.0 g H
2
O: are needed to react completely
CaC
2
+ 2 H
2
0

C
2
H
2
(g) + Ca(OH)
2
How many moles of CaC
2 are needed to react 49.0 g of H
2
0?
CaC
2
+ 2 H
2
0

C
2
H
2
(g) + Ca(OH)
2
? moles 49.0g
Units match
49 g H
2
O 1 mol H
2
O
18 .
0 g H
2
O
1
2 mol mol
CaC
H
2
O
2
2

Stoichiometry Review Problem
Suppose you want to figure out how many grams of ammonia you can produce if you react 60.0 g of hydrogen gas with excess nitrogen.
Remember the balanced chemical equation gives the mole relationship between reactants and products.
N
2
+ 3 H
2
2 NH
3
1) Take the grams of hydrogen gas (recall that it is diatomic) and convert it to moles:
2) Convert the moles of hydrogen gas to moles of ammonia, using the coefficients in the balanced chemical equation:
3) Convert from moles of ammonia to grams of ammonia using the molecular mass and solve the equation:

Calculating the Percent Yield
 The predicted amount of product – which we have been doing with our stoichiometry problems has been for 100% yield, or the
theoretical yield of a reaction.
 The theoretical yield is the maximum amount of product that can be produces in a given reaction.
 When chemical reactions take place, they are almost never 100% complete. A reaction may not go to 100% due to not all the reactants becoming involved, impurities in the reactants, competing side reactions, loss of product due to filtering, or just not getting it all out of the vessel.
 The actual yield is how much product can be collected – and measured.
 The percent yield is a ratio given by:
Percent Yield = Actual Yield x 100%
Theoretical Yield

Calcium carbonate is decomposed by heating, as shown in the following equation:
C
2
1) What is the theoretical yield of this reaction if 24.8 g CaCO
3 is heated to yield 13.1 g CaO?
Handle like a typical stoichiometry problem: grams to grams
Ignore the 13.1 g CaO to solve. This is the actual yield.
2) What is the percent yield?

Chlorobenzene is used in the production of many different chemicals, such as aspirin, dyes and disinfectants. One method of preparing chlorobenzene is to react benzene C
6
H
6
, with chlorine according to the following equation:
C
6
H
6
+ Cl
2
C
6
H
5
Cl + HCl
1) When 36.8 g of C
6
H
6 react with excess of Cl
2
, the actual yield of C
6
H
5
Cl is 38.8g?
What is the theoretical yield?
Handle like a typical stoichiometry problem: grams to grams
2) What is the percent yield?

Equal volumes of all gases at the same temperature and pressure contain the same number of
molecules.
Amedeo Avogadro
At STP (Standard Temperature and Pressure):
1 mole of a gas occupies 22.4 liters of volume

If reactants and products are at the same conditions of temperature and pressure, then mole ratios of gases are also volume ratios.
3 H
2
(g) + N
2
(g)  2NH
3
(g)
3 moles H
2
3 liters H
2
+ 1 mole N
2
 2 moles NH
3
+ 1 liter N
2
 2 liters NH
3

How many liters of ammonia can be produced when 12 liters of hydrogen react with an excess of nitrogen?
3 H
2
(g) + N
2
(g)  2NH
3
(g)
12 L H
2
2
3
L NH
3
L H
2
8.0
3

How many liters of oxygen gas, at STP, can be collected from the complete decomposition of 50.0 grams of potassium chlorate?
2 KClO
3
(s)  2 KCl(s) + 3 O
2
(g)
50.0 g KClO
3
1 mol KClO
3
122.55 g KClO
3
3 mol O
2
2 mol KClO
3
22.4 L O
2
1 mol O
2
13.7
2