Stoichiometry of Chemical Reactions
Stoichiometry refers to the calculation of the quantities of reactants and products involved in a chemical reaction. Stoichiometry is perhaps the most important topic in general or introductory chemistry and one that must be mastered if the student is to be succesful in the course. Stoichiometric calculations must be based on a balanced chemical equation, so before you start punching numbers into your calculator make sure the equation is balanced. A stoichiometry problem will involve only two substances in the balanced equation at one time. One of the substances is referred to as the given substance, the problem will give the amount of this substance either as mass, moles, or perhaps volume. The other substance in the problem is referred to as the desired substance, usually the problem is seeking the amount of this substance, in moles, mass or volume. To summarize:
| Step 1. Write a balanced equation for the reaction or check to see that a given equation is balanced. |
| Step 2. Use molar mass to convert mass of given substance to moles. |
| Step 3. Obtain a stoichiometric factor from the equation to convert moles of given substance to moles of desired substance. |
| Step 4. Use molar mass to convert moles of the desired substance to mass of that substance. |
In some problems all of the four steps may not be neccessary, for example if the problem gave the quantity of the given substance in moles and asked for the quantity of the desired substance in moles, steps 2 and 4 would not be needed. So here, as always, you should read the problem carefully. Next we will work through an example step by step.
Example problem
How much iron(III) oxide will be produced by the complete oxidation of 101 g of iron according to the following equation? 4 Fe + 3 O2 ==> 2 Fe2O3?
Solution
From the question we deduce that Fe is the given substance and that Fe2O3 is the desired substance.
| Step 1. The equation is already balanced. |
| Step 2. Convert the mass of Fe to moles using the molar mass of Fe. |
1 mol Fe |
||||
101 g Fe |
x |
------------------ |
= |
1.81 mol Fe |
55.85 g Fe |
| Step 3. Use coefficients from the balanced equation to obtain a factor that relates moles of Fe to moles of Fe2O3. |
2 mol Fe2O3 |
||||
1.81 mol Fe |
x |
------------------ |
= |
0.905 mol Fe2O3 |
4 mol Fe |
| Step 4. Convert the moles of Fe2O3 to grams using the molar mass of of Fe2O3. |
159.7 g Fe2O3 |
||||
0.905 mol Fe2O3 |
x |
------------------ |
= |
145 g Fe2O3 |
1 mol Fe2O3 |
All of these steps can be combined into a single equation as follows.
1 mol Fe |
2 mol Fe2O3 |
159.7 g Fe2O3 |
||||||
101 g Fe |
x |
------------------ |
x |
------------------ |
x |
------------------ |
= |
1145 g Fe2O3 |
55.85 g Fe |
4 mol Fe |
1 mol Fe2O3 |