# Introduction to stoichiometry

### Introduction to stoichiometry

#### Lessons

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.

Notes:
• The coefficients (the number before the chemical symbol / formula) tell you how many molecules of that chemical are used, as a ratio, in the reaction. If no number is there, it’s one molecule.
• Recall from lesson on Introduction to chemical reactions that a chemical equation tells you the fixed ratio of reactants the reaction uses, and products that the reaction produces.
$\circ$ In the made up example below, we are being told that to produce 2 molecules of C and 1 molecule of D, the reaction requires 1 molecule of A and 2 molecules of B IN THAT RATIO.
• A $+$ 2B → 2C $+$ D

• The actual amounts of reactants used or products made in a particular reaction can be changed, but the ratio between them cannot – changing plans to make twice as much of a product will need twice as much reactants. There is no way around this fact!
• A mole is just a number of atoms – $6.02 \times 10^{23}$ (six hundred and two billion trillion!) but that is it - it is just a number in exactly the same way that a dozen or a pair is a number. It is used in chemistry because the mass of atoms and molecules are measured in grams per mole (written gmol$^{-1}$).
• Take carbon dioxide, CO$_2$ as an example. CO$_2$ has a molecular mass of 44 grams per mole. That means that 1 mole (six hundred billion trillion molecules) of carbon dioxide has a mass of 44 grams. Similarly, a sample of 22 grams of CO$_2$ would be half a mole of CO$_2$ (as 22g is half of 44g).
• The relative mass, measured in grams per mole is vitally important for chemists because atoms and molecules are too small to be counted individually but we can measure the mass of substances very easily! With the mass and grams per mole known, we can work out the number of moles – how many molecules are in the sample of the substance we are using.
• Use the chemical equation to find the ratio of reactant moles to product moles (the reaction stoichiometry) and then find their relative masses. Now, you can predict the mass of any reactants required or products expected for any reaction scale.
• Introduction
Introduction to Stoichiometry
a)
What is Stoichiometry?

b)
What information does a chemical equation tell us?

c)
What is a mole in chemistry?

• 1.
Find the ratio of molecules used and produced in a process: the reaction stoichiometry.
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$_2$?

• 2.
Find the reaction stoichiometry and use it to predict expected mass of products.
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?

• 3.
Find the reaction stoichiometry and use it to predict the mass of chemicals used in a reaction.
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?

• 4.
Find the reaction stoichiometry and use it to predict the mass of chemicals used and produced in a reaction.
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?