# Introduction to stoichiometry

### Introduction to stoichiometry

#### Lessons

• The coefficients (the number before a chemical formula) tell you how many molecules of that chemical are used in the reaction. If no number is there, it’s one molecule.

• A chemical equation tells you the fixed ratio of molecules of reactants the reaction uses, and products that it produces.

• The actual amounts of reactants used or products made in a reaction can be changed but the ratio between them does not – making twice as much of a product will require twice as much reactants!

A mole is a precise number of atoms$6.02 \times 10^{23}$ (six hundred billion trillion!) and it’s used because atomic and molecular masses are measured in grams per mole (written gmol$^{-1}$).

• Use the chemical equation to find the ratio of reactant moles to product moles and then find their relative masses. Now, you can predict the mass of any reactants required or products expected for any reaction scale.

In this lesson, we will learn:
• How to analyze a balanced chemical equation.
• What a “mole” is and how it is important to understanding chemical reactions.
• How to predict amounts of product made in a reaction with a given amount of reactant.
• How to apply the moles concept with conversion factors to find the mass of reactants and products involved in reactions.
• 1.
Introduction to Stoichiometry
a)
What is Stoichiometry?

b)
What information does a chemical equation tell us?

c)
What is a mole in chemistry?

• 2.
Discuss
2H$_2+$O$_2$→2H$_2$O
a)
How many molecules of H$_2$ will react with 6 molecules of O$_2$?

b)
How many molecules of H$_2$O would this produce?

c)
What is the mass of 6 moles of O$_2$?

• 3.
CH$_4 +$2O$_2$→CO$_2 +$2H$_2$O
a)
How many moles of H$_2$O were produced if 2.5 moles of CO$_2$ were produced?

b)
What is the mass of 2.5 moles of CO$_2$?

c)
In a repeat experiment, 132 grams of CO$_2$ were produced. What was the mass of water also produced?

• 4.
HCI$+$NaOH→NaCl$+$H$_2$O
a)
How many moles of HCI are required to react with 4.75 moles of NaOH?

b)
What would be the mass of 4.5 moles of NaOH?

c)
In a repeat experiment, 54.75g HCI (dissolved in solution) reacted completely with some NaOH added. What was the mass of NaOH used in this experiment?

• 5.
2C$_2$H$_6 +$7O$_2$→4CO$_2 +$6H$_2$O
a)
How many moles of O$_2$ would react with 1 mole of C$_2$H$_6$?

b)
If 15 moles of H$_2$O were produced in this experiment, how many moles of C$_2$H$_6$ were used?

c)
What is the mass of this amount of C$_2$H$_6$ used?

d)
A total of 0.19 mol of reactants and products combined were involved in this reaction. What is the number of moles of H$_2$O produced?