Stoichiometry

Introduction to Stoichiometry

Mole-to-Mole Ratios

Stoichiometry

Learning Target 4.6.1 : I can use Law of Conservation of Mass to describe and calculate mole-to-mole ratios from a balanced chemical equation.

Common Misconceptions:

• Students might relate coefficients in a balanced chemical equation to mass ratios instead of to mole ratios. Use of coefficients to directly compare masses does not always correctly reflect the law of conservation of mass. Mole ratios tell us how many atoms or compounds/molecules are necessary/produced for the reaction to occur during common situations.

Introduction to Stoichiometry

After having put together everything we have learned thus far in Chemistry, we are going to explore the usefulness of a fully balanced chemical equation. To start, let’s have S’MORE^[1] Chemistry practice!

^[2]S’Mores!

Stoichiometry, stoi·chi·om·e·try, pronounced: /ˌstoikēˈämitrē^[3] is the study and calculation of quantitative (measurable) relationships of the reactants and products in chemical reactions (chemical equations).^[4] 

As an introduction, please watch the first 2 minutes and 50 seconds (2:50) of this stoichiometry video^[5] by Khan Academy.

Mole-to-Mole Ratios

Goal 4.6.1a: I can use a balanced chemical equation to identify mole-to-mole ratios.

A ratio is a comparison of how much or how many of one thing to another thing.

So, if I have 12 eggs and 1 carton, I would have 1 dozen eggs.

12 Eggs + Carton → Dozen (Eggs)

I can say that:

For every 12 Eggs, I have 1 Carton.

For every 1 Carton, I have 12 Eggs.

For every 1 Carton, I have 1 Dozen (Eggs).

For every 1 Dozen (Eggs), I have 1 Carton.

Finally, for every 12 Eggs, I have 1 Dozen (Eggs).

For every 1 Dozen (Eggs), I have 12 Eggs.

All six of these ratios are comparing how many Eggs, Cartons, and Dozen(Eggs).

You try!

If you are going to make ice cream, you would follow this recipe:

3cup cream

1 cup sugar

churned over ice and salt

makes 2 cups ice cream

If we had to put this into a chemical equation, it would look something like this:

                                 churn with

                                ice & salt

3c cream + 1c sugar        2c ice cream

On a separate sheet of paper, write all the possible ingredient ratios that exist in this recipe.

Check your ratios at the bottom of this lesson.

Now, correlate this idea to a balanced chemical equation.

2H2(g)  + O2(g)  →  2H2O(g)

I can say that:

For every 2 moles of H2(g), I have 1 mole of O2(g).

For every 1 mole of O2(g), I have 2 moles of H2(g).

For every 1 mole of O2(g), I have 2 moles of H2O(g).

For every 2 moles of H2O(g), I have 1 mole of O2(g).

Finally, for every 2 moles of H2O(g), I have 2 moles of H2(g).

For every 2 moles of H2(g), I have 2 moles of H2O(g).

^[6]Take Note!

Notice that the coefficient used to balance the chemical equation is our mole ratio!

Goal 4.6.1b: I can use mole-to-mole ratios to calculate molar relationships in a chemical reaction.

We can use mole ratios from a balanced chemical equation to do calculations about all substances in a reaction.

Using the chemical reaction above, 2H2(g)  + O2(g)  →  2H2O(g) consider the following example question.

Example 1:

If I use 6.7 moles of Oxygen (O2(g)), how many moles of Water (H2O(g)) are created?

Always start a stoichiometric calculation with information given in the problem.

6.7mol O2  x   2mol H2O   =  13.4 = 13mol H2O are created when using 6.7mol O2.

                   1mol O2

Since you start with mol O2 and multiply by something over mol O2, mol O2 cancel out. This is a critical part of dimensional analysis, and equally critical for stoichiometry.

Considering significant figures, you only use the measurement of 6.7mol to determine how many digits can go in your answer. The numbers 2 and 1 from the mole ratio are not measurements and therefore not considered for significance.

For more examples of how to do stoichiometric calculations using mole-to-mole ratios, please watch this video^[7]by Boundless Chemistry.

Then, please try for yourself in this Mole-to-Mole Practice Worksheet^[8].

When you are done, please check your answers using this KEY^[9].

Please be sure to check your work, including unit cancellation and the final answers’ units as well as significant figures.

How did you do? Rate yourself on your ability to do Mole-to-Mole Conversions:

3= I can do Mole-to-Mole Conversions confidently and am ready to move on.

2= I can do Mole-to-Mole Conversions hesitantly and I should do more practice^[10].

1= I need to redo this lesson and try Mole-to-Mole Conversions again to better understand.

Check your ratios HERE: you should have found 6.

                                 churn with

                                 ice & salt

3c cream + 1c sugar        2c ice cream

For every 3c cream, I have 1c sugar

For every 1c sugar, I have 3c cream

For every 3c cream, I have 2c ice cream

For every 2c ice cream, I have 3c cream

For every 1c sugar, I have 2c ice cream

For every 2c ice cream, I have 1c sugar