What you will need to know how to do to be successful at stoich:
1. Dimensional analysis
2. Calculating molar mass
3. Balancing equations
4. Using coefficients to determine mole ratios.
Learning Objectives:
1. Give the mass or amount of any substance in a chemical reaction, calculate the mass or amount of any other substance in the reaction.
2. Solve limiting reactant problems.
3. Solve percent yield problems.

Intro : Stoichiometry is the method used by chemists to calculate the amounts of substances needed or produced in chemical reactions.
It is an application of dimensional analysis using:
A. Molar masses to convert between grams and moles.
B. The coefficients from a balanced equation to convert between moles of different substances.

• One plus one isn’t always two

In Math class you often use Pi

But not that kind of Pie in chemistry

+
5 Apples + 1 Crust
↑ ↑
1 Pie
↑
The coefficients are ratios of amounts

Ratio of apples to crusts =
Ratio of apples to pies =
Ratio of crusts to pies =

It works the same in chemistry. The coefficients are mole ratios.
2NaN
3
2Na + 3N
2
6Na + Fe
2
O
3
2Fe + 3Na
2
O
Na
2
O + 2CO
2
+ H
2
O → 2NaHCO
3
But why would we need to know exact amounts of reactants or products?

The Reactions Prevent This:

The gas in an airbag is not air, but nitrogen

• Not Enough N
2
Too much N
2

3
→
2
To get the correct amount of N
2 the correct amount of NaN
3 we must start with
. Stoich is used to calculate this amount.

Back to Pies!
+
5 Apples + 1 Crust
1 Pie

I want 2 pies! How many crusts? How many apples?
# crusts = 2 pies 1 crust
1 pie
= 2 crusts
#Apples =
2 pies 5 apples
1 pie
= 10 Apples

Let’s get chemical!
Any balanced chemical reaction will give you the mole ratios between all substances in the reaction.
N
2
+ 3H
2
→ 2NH
3
What is the mole ratio of N
2 to H
2
?
What is the mole ratio of N
2 to NH
3
?
What is the mole ratio of H
2 to NH
3
?

We may use these ratios as conversion factors to convert between any two substances in the reaction – just as we converted between pies and crusts and apples.
N
2
+ 3H
2
→
2NH
3
If we have 4 moles of H
2 produced?
Moles NH
3
=
4 moles H
2
, how much NH
3 will be
2 moles NH
3
3 moles H
2
If we start with 0.2 moles of N
2
Moles H
2
=
0.2moles N
2
,how much H
3 mole H
2
2
1 moles N
2 is needed?

Practice: Given the following reaction,
4Al + 3O
2
→ 2Al
2
O
3
If we start with 1 mole of Al, how much Al
2
O
3 produced?
will be
How much Al will react with 0.4 moles of O
2
?
How much O
2
Al
2
O
3
?
will be required to produce 5 moles of

Note that it does not matter whether you are given an amount of a product or reactant. The coefficients give you mole ratios for all substances!
The number of moles of a substance is usually difficult to measure directly in the lab. It is much easier to measure the amount of a substance by mass.
Therefore, in stoich problems you are usually given the mass of a substance rather than the number of moles and asked to calculate the mass of another substance in the reaction. These are called mass- mass problems.

Solving mass-mass problems:
You will be given the mass of one substance in a reaction (substance A) and asked to calculate the mass of a different substance in the reaction
(substance B).
This will be a 3-step dimensional analysis conversion.
1. Convert grams of A to moles of A using the molar mass of A.
2. Convert moles of A to moles of B using the coefficients from the balanced equation.
3. Convert moles of B to grams of B using the molar mass of B.

Flow chart for mass-mass problems
Given grams of substance A
Convert to grams of A to moles of A using molar mass of A
Convert moles of A to moles of substance B using coefficients from the balanced equation
Convert moles of B to grams of B using molar mass or of B

Ex: Given the following reaction, how many grams of
N
2 will be produced by the reaction of 5.0g of
NaN
3
?
2NaN
3
Given: 5.0 grams of NaN
3
→ 2Na + 3N
2 molar mass = 65 g/mol
Looking for: ? grams of N
2 molar mass = 28 g/mol
Conversion: 2 mol NaN
3
/ 3 mol N
2
5.0g NaN
3
1 mole NaN
3
65 g NaN
3
3 mole N
2
2 mole NaN
3
28 g N
2
1 mole N
2

Ex2: If 12.5 g Of Na are present, how many grams of iron
(III) oxide will be required for a complete reaction?
6Na + Fe
2
O
3
Given: 12.5g Na
→ 2Fe + 3Na
2
O
Molar mass=23 g/mol
Need: ? g Fe
2
O
3
Molar mass=159.8 g/mol
Conversion: 6 mol Na / 1 mol Fe
2
O
3
12.5g Na 1 mole Na 1 mole Fe
2
O
3
23 g Na 6 mole Na
159.8g Fe
1 mole Fe
2
O
3
2
O
3

There are only 4 possible variations to this type of problem:
1. Given moles of A : looking for moles of B
- 1 step conversion using mole ratio.
2. Given moles of A : looking for grams of B
- 2 step conversion
Use mole ratio to convert moles of A to moles of B.
Use molar mass of B to convert moles of B to grams of B.

3. Given grams of A : looking for moles of B
- 2 step conversion
Use molar mass of A to convert grams of A to moles of A.
Use mole ratio to convert moles of A to moles of B.
4. Given grams of A : looking for grams of B
- 3 step conversion
This is a mass-mass problem

Practice: For the reaction:
Na
2
O + 2CO
2
+ H
2
O → 2NaHCO
3
If you start with 0.35 moles of CO
2 of H
2
O are needed?
, how many grams
If you have 100g of Na
2 react?
O, how many moles of CO
2
If 37.5g of NaHCO
3
Na
2
O reacted?
are produced, how many grams of

Limiting Reactants: Are reactants in a chemical equation that are used up before any other reactant.
When this reactant is used up, the reaction must stop.
Think about making pies!
5 apples + 1 crust → 1 pie
What if you have 16 apples and 2 crusts. How many pies can be made?
The ingredient (reactant) that runs out and stops the reaction is the limiting reagent. The reactant that is left over is said to be “in excess”.

It works the same way in chemistry. In any real reaction, you will be adding real amounts of reactants to each other. One of them will nearly always run out first.

How to solve limiting reactant problems:
In this type of problem, you will be given masses of two different reactants and asked to determine which is the limiting reagent and how much of a given product is produced.
Do a mass-mass problems starting with reactant 1 and looking for the mass of the asked for product.
Do a 2 nd mass-mass problems starting with reactant
2 and looking for the mass of the same product.
Compare the calculated masses. The smaller of the
2 masses is the amount produced and the reagent that gave that answer is limiting.

Ex: For the reaction:
N
2
+ 3H
2
→ 2NH
3
If you start with 5.0g of N
2 and 3.0g of H
2 the limiting reactant and how much NH
3
, what is is produced?
Mass-mass 1:
Mass-mass 2
Compare:

Practice:
6Na + Fe
2
O
3
→ 2Fe + 3Na
2
O
Given the above reaction, if you start with 20.0g of Na and 15.0g of Fe
2
O
3
, how much Fe will be formed?

Percent yield problems.
Not all chemical reactions go to completion. Even for those that do, it is often difficult to recover all of the product of interest.
Ex:
Percent yield is a way to express the amount of product that is actually obtained in a lab compared to the theoretical amount predicted by a mass-mass calculation.
It is defined as:
%yield =
Actual mass x 100
Theoretical mass

Practice:
For the reaction: 2Al
2
O
3
→ 4Al + 3O
2
, the amount of Al expected from a mass-mass calculation was 24.5g. The amount collected in the lab was 22.7g. What is the %yield?
If you started with 50g of Al
2
O
3
Al, what is the % yield?
and obtained 8g of