Stoichiometry

Just what is stoichiometry?

The word stoichiometry is derived from two Greek words: stoicheion (meaning element) and metron (meaning measure).

Why do we use stoichiometry?

Whenever chemists want to know how much of a desired product will be produced, they start with a balanced equation.

How does it work?

Then the chemist uses the equation to guide them in calculating the products. Since stoichiometry is used to predict things in the “perfect world”, we can expect that the reaction in real life won’t create the maximum amount of product.

The chemical equation is the chemist’s “recipe”!

By using the chemical equation, chemists can calculate the amount of materials needed or the approximate amount of materials produced (the amount of materials actually produced in the lab is approximate since we do not live in a “perfect” world).

How do I use the chemical equation?

The chemical equation is written like a mathematical expression. When balanced, the coefficients tell how many moles of each reactant or product is needed or expected.

Example:

2H2 + O2 2H2O

One way to understand the information conveyed by a chemical equation is to convert the equation into an English sentence.

So: 2H2 + O2 2H2O

This equation really says: “two moles of H2 added to one mole O2 yields two moles of water.

(the coefficients tell how many moles, atoms or molecules of each chemical are needed in the “recipe”)

Quantitative Information from Balanced Equations

In addition to being both a qualitative and quantitative statement of fact, a balanced chemical equation is an algebraic equality in which the arrow is the equal sign. This means that we can manipulate a balanced chemical equation in much the same way as we can manipulate any other algebraic equality.

Consider the combustion of propane.

C3H8 + 5 O2 3CO2 + 4H2O

Stoichiometry has 5 basic steps:

Step 1: Write and balance the equation

Step 2: Write in all information given (make sure to identify what you are trying to find!)

Step 3: Convert everything to moles

Step 4: Use mole ratio to solve for what you are trying to find (you may also use a proportion to solve the problem.)

Step 5: Convert everything into the required unit if needed.

What is a mole ratio?

Mole Ratios: The mole ratio is based on the balanced chemical equation.

Example of a mole ratio:

 

CH4 + 2O2 CO2 + 2H2O

 

The mole ratio of CH4:O2 is 1:2. Another

way to write this is CH4.

2O2

How do you use a mole ratio?

The mole ratio is an equivalent which means that you can arrange the ratio in any way needed.

Example of a mole ratio problem:

N2 + 3H2 2NH3

 Write the molar ratios for N2 and H2.

Write the molar ratios for NH3 and H2.Convert the ratios above into a

fraction.

Examples of a mole-mole stoichiometric problem:

2H2 + O2 2H2O

 

1. How many moles of H2O are

produced when 5 moles of oxygen are used?

Examples of a mole-mole stoichiometric problem continued:

2H2 + O2 2H2O

2. If 3.00 moles of H2O are produced,

how many moles of oxygen must be consumed?

Examples of a mole-mole stoichiometric problem continued:

2H2 + O2 2H2O

3. How many moles of hydrogen gas must be used, given the data in problem #2.

Example of a mole-mass stoichiometric problem:

2H2 + O2 2H2O

If 3.00 moles of oxygen are completely converted to water, what is the mass of the water produced?