Stoichiometry is the area of chemistry which deals with the quantitative relationship among reactant and products. We will use a unit analysis of moles and molar mass to solve problems in stoichiometry. A mol-ratio is a ratio between any two species in a chemical equation.

For example in the equation

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

there are a number of relationship implicit in the balanced equation. 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. Beginning with 2 moles of potassium peroxide require 1 mol of carbon dioxide for complete reaction. 12 moles of KO2 requires 6 mol of carbon dioxide. The ratio is always maintained.

This ratio can be expressed from the balanced chemical equation as the following unit conversion,

The unit conversion 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.

Sample Problem #1:

In the laboratory one might weigh amounts using the balance. Let's consider another example,

Sample Problem #2:

Sample Problem #3:

Sample Problem #4: