Stoichiometry is the area of chemistry which deals with the quantitative relationship among reactant and products in a balanced chemical equation. Here is a balanced chemical equation for our consideration,

4KO2(s) + 2CO2(g) ---> 2K2CO3(s) + 3O2(g)

To begin with we must be able to 'read' this chemical equation. It can be read two ways. One way is as follows: 4 formula units of KO2 reacts with 2 molecules of carbon dioxide to produce 2 formula units of potassium carbonate and 3 molecules of oxygen. The coefficients are simply numbers of each substance. We use the term formula unit to describe an ionic compound and the term molecule to describe a covalent compound.

According to the chemical equation

4KO2(s) + 2CO2(g) ---> 2K2CO3(s) + 3O2(g)

a) how many molecules of carbon dioxide react with two formula units of KO2? Answer

b) how many molecules of dioxygen are produced when eight formula units of KO2 react? Answer

c) how many formula units of potassium carbonate are produced when six molecules of CO2 react with excess potassium superoxide? Answer

d) how many formula units of potassium superoxide must react to form 27 molecules of O2 assuming excess carbon dioxide? Answer

The balanced chemical equation can also be 'read' in terms of mole. For example, the equations says that 4 moles of KO2 reacts with 2 moles of carbon dioxide to produce 2 mol of potassium carbonate and 3 moles of oxygen. The ratio of these amount is always maintained in a chemical reaction. These ratios, or stoichiometric ratios are derived from the balanced chemical equation and are used to convert from one substance in the chemical equation to another. Examples of these stoichiometric ratios using the above reaction between potassium superoxide and carbon dioxide are;

and

These stoichiometric factors comes from the balanced chemical equation and can be used to determine amounts required for reaction. For example to determine how many moles of carbon dioxide are required to react with 2 moles of potassium superoxide we could perform the following conversion,

There are several additional mol ratios which derive from the equation, a few include,

These ratios are used as conversion factors to convert from moles of one substance to the corresponding moles of another substance in a chemical rxn. Learning to do stoichiometric calculations will require practice. One nice thing about these calculations is the step-by-step procedure that can be followed to arrive at an answer. The step-by-step procedure is as follows;

  1. Balance the chemical equation which describes the chemical reaction.

  2. Use the molar mass of the reactant to convert the grams of reactant to moles.

  3. Use the unit conversion (mole ratio) from the balanced chemical equation to convert from moles reactant to moles product.

  4. Use the molar mass of the product to convert from moles to grams.

Lets demonstrate a variety of stoichiometric calculations that you will be responsible for by beginning with several straight forward uses of mol-ratios.

According to the chemical equation

2C2H6(g) + 7O2(g) ---> 4CO2(s) + 6H2O(g)

a) how many mol of carbon dioxide will form by the complete combustion of 6.6 mol of C2H6? Answer

b) how many mol of carbon dioxide will form by the complete combustion of 0.410 mol of C2H6? Answer

c) how many mol of ethane, assuming excess dioxygen, are required to form 3.7 mol of water? Answer

d) how many mol of dioxygen are required to produce with 5.67 x 10-3 mol CO2? Answer

These last four stoichiometry conversion problems used only moles. In the laboratory the amount of a substance in a chemical reaction is weighed using the balance. Let's consider another example,

According to the chemical equation

2CuCl2(s) + 4KI(s) ---> 2CuCl(s) + 4KCl(s) + I2(s)

a) how many grams of iodine are produced when 145 g of KI react with excess copper(II) chloride? Answer

b) how many grams of copper(II) chloride are required to react with 145 g of KI? Answer

c) how many grams of copper(II) chloride must react, assuming excess KI, to form 8.11 x 10-3 grams potassium chloride? Answer

Here is another interesting kind of stoichiometry question.

The equation for the reaction is

2S(g) + 3O2(g) ---> 2SO3(g)

Consider a mixture of sulfur atoms and dioxygen molecules in a closed container below:

For each of the following explain why the representation is correct or incorrect. (Click on a diagram for an answer.)