Download mass-mass problems.

Chapter 10 Stoichiometry
Or
One plus One isn’t always Two
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.
2NaN3 → 2Na + 3N2
6Na + Fe2O3 → 2Fe + 3Na2O
Na2O + 2CO2 + H2O → 2NaHCO3
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 N2
Too much N2
The nitrogen is produced by this
reaction.
2NaN3 → 2Na + 3N2
To get the correct amount of N2 we must start with
the correct amount of NaN3 . 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 = 2 crusts
1 pie
#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.
N2 + 3H2 → 2NH3
What is the mole ratio of N2 to H2?
What is the mole ratio of N2 to NH3?
What is the mole ratio of H2 to NH3?
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.
N2 + 3H2 → 2NH3
If we have 4 moles of H2 , how much NH3 will be
produced?
4 moles H2 2 moles NH3
Moles NH3 =
3 moles H2
If we start with 0.2 moles of N2 ,how much H2 is needed?
Moles H2 = 0.2moles N2 3 mole H2
1 moles N2
Practice: Given the following reaction,
4Al + 3O2 → 2Al2O3
If we start with 1 mole of Al, how much Al2O3 will be
produced?
How much Al will react with 0.4 moles of O2?
How much O2 will be required to produce 5 moles of
Al2O3?
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 massmass 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
N2 will be produced by the reaction of 5.0g of
NaN3?
2NaN3 → 2Na + 3N2
Given: 5.0 grams of NaN3 molar mass = 65 g/mol
Looking for: ? grams of N2 molar mass = 28 g/mol
Conversion: 2 mol NaN3 / 3 mol N2
5.0g NaN3 1 mole NaN3 3 mole N2
28 g N2
65 g NaN3
2 mole NaN3 1 mole N2
Ex2: If 12.5 g Of Na are present, how many grams of iron
(III) oxide will be required for a complete reaction?
6Na + Fe2O3 → 2Fe + 3Na2O
Given: 12.5g Na
Molar mass=23 g/mol
Need: ? g Fe2O3 Molar mass=159.8 g/mol
Conversion: 6 mol Na / 1 mol Fe2O3
12.5g Na
1 mole Na
23 g Na
1 mole Fe2O3
6 mole Na
159.8g Fe2O3
1 mole Fe2O3
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:
Na2O + 2CO2 + H2O → 2NaHCO3
If you start with 0.35 moles of CO2, how many grams
of H2O are needed?
If you have 100g of Na2O, how many moles of CO2
react?
If 37.5g of NaHCO3 are produced, how many grams of
Na2O reacted?
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 2nd 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:
N2 + 3H2 → 2NH3
If you start with 5.0g of N2 and 3.0g of H2 , what is
the limiting reactant and how much NH3 is
produced?
Mass-mass 1:
Mass-mass 2
Compare:
Practice:
6Na + Fe2O3 → 2Fe + 3Na2O
Given the above reaction, if you start with 20.0g of Na
and 15.0g of Fe2O3 , 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: 2Al2O3 → 4Al + 3O2 , 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 Al2O3 and obtained 8g of
Al, what is the % yield?