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Q U A N T I T A T I V E R E L A T I O N S H I P S I N C H E M I S T R Y
Stoichiometry
What Is Stoichiometry?
It is the study of the quantitative relationships among reactants and products in a chemical reaction
It’s All in the Numbers!
The relationships among reactants and products in a chemical reaction are given by the coefficients in a balanced chemical equation
2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O
It’s All in the Numbers!
The coefficients can be interpreted as the number of molecules or moles of each substance
2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O
It’s All in the Numbers!
We will usually interpret coefficients as the amount of moles of each substance since that is more practical in lab work
2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O
It’s All in the Numbers!
The balanced equation below shows that:
2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O
2 moles of octane
need to react with 25 moles of oxygen
to produce 16 moles of carbon dioxide
and 18 moles of water
Stoichiometry Allows Predictions
How would the other coefficients change if you doubled the moles of octane to 4 instead of 2?
2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O 4 __ __ __
Each Coefficient is Related to the Others
They would double also because every coefficient in a balanced chemical equation is proportionally related to the others
2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O 4 50 32 36
In Chemistry, We Use “Mole Ratios”
A mole ratio is a fraction that allows us to compare moles of one substance to moles of another substance based on the coefficients 2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O
What Do We Do With Mole Ratios?
We use mole ratios as conversion factors in dimensional analysis problems to help us predict…
how much product can be made or how much reactant is needed during a chemical reaction
Let’s see how….
Example 1: A “Mole-to-Mole” Problem
How many moles of water are produced from burning 37.0 moles of octane?
Step 1: Write a balanced equation
2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O
Example 1: A “Mole-to-Mole” Problem
How many moles of water are produced from burning 37.0 moles of octane?
Step 2: Convert moles of “substance A” into moles of “substance B” (Convert moles of octane into moles of water) 2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O 37.0mol C 8 H 18 x 18 mol H 2 O = 333 mol H 2 O 2 mol C 8 H 18
This “mole ratio” allowed us to convert from octane to water
In Summary
According to the balanced reaction,
Burning 312 grams of octane will result in the production of 443 grams of water.
2 C 8 H 18 + 25 O 2 16 CO 2 + 18 H 2 O
Summary of Stoichiometry Problems
- Write a balanced chemical equation
- Convert given substance “A” into moles
- Convert moles of “A” into moles of “B” (mole/mole ratio)
- Convert moles of substance “B” into desired units
Why Learn Stoichiometry?
It allows you to determine… The amount of reactant(s) you need and The amount of product(s) you will make
