Chemistry Unit 7 - Stoichiometry Day 1 Notes

Using chemical equations (mole Ratios)

I can...

• determine the relationships between moles of reactants and products using information from a balanced chemical equation.

Success criteria

I know...

• how to use the information given to me in a balanced chemical equation.

Learning Target

Stoichiometry

Stoichiometry is just a fancy word to describe how a chemist can use equations to calculate amounts in reactions.

• It uses the coefficient ratio of balanced equations to make connections between the reactants and products in reactions.

• The equation must be balanced!

We already have the tools necessary to solve any question about the amounts in reactions. We just need to learn a new way to apply skills we have acquired.

First you need to recall...

Reactions are described by equations that give us the identities of our reactants and products. They also show us how much of each reactant and product are involved in the reaction.

• A balanced chemical equation gives relative numbers (or moles) of reactant and product molecules that participate in a chemical reaction.

• The coefficients of a balanced equation give the relative numbers of molecules.

Assume that you have a job at a deli that has a specialty sandwich made out of 2 pieces of bread, 3 slices of meat, and 1 slice of cheese. The sandwich process can be represented in an equation like this:

Consider this...

​2 breads + 3 meats + 1 cheese → 1 Sandwich

One day a customer orders 50 specialty sandwiches. How much of each reactant (ingredient) do you need?

We can use the mole to mole ratio of reactants and products to help with this. Based on the recipe:

​2 breads + 3 meats + 1 cheese → 1 Sandwich

• if you have 50 sandwiches:

  • you will use 2 x 50 slices of bread = 100 bread slices

    • this is because the bread to sandwich ratio is 2:1

  • you will use 3 x 50 slices of meat = 150 meat slices

    • this is because the meat to sandwich ratio is 3:1

  • you will use 1 x 50 slices of cheese = 50 cheese slices

    • this is because the cheese to sandwich ratio is 1:1

The answer...

how can we use these types of ratio relationships?

Similar to the sandwich scenario, chemical equations that are balanced will give you a ratio of reactant and product molecules.

COEFFICIENTS ARE THE KEY TO STOICHIOMETRY.

• We can use those ratios from the coefficients to determine the amount of reactants needed to give a certain amount of product or to predict how much product can be made from a given quantity of reactants.

​

Propane, C₃H₈, is a fuel commonly used for cooking on gas grills and for heating in rural areas where natural gas is unavailable. Propane reacts with oxygen gas to produce heat and the products carbon dioxide and water. This combustion reaction is represented by the balanced equation:

C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)

Determine the mole to mole ratio of the reactants and products.

​Example 1: relating moles to molecules

​ANSWER:

• 1 mol C₃H₈

• 5 mol O₂

• 3 mol CO₂

• 4 mol H₂O

An example of a ratio is:

• 5 moles of O₂ will help yield 4 moles of H₂O or...

  • 5 mol O₂ : 4 moles of H₂O

Multiple Choice

This equation tells us that 2 moles of H₂O will yield ___ moles of H₂.

Equation: 2H₂O → 2H₂ + O₂

• 1

• 2

• 3

• 4

​

How much O₂ will be produced by a reaction that begins with 5.8 moles of water (H₂O)?

​Example 2: using mole ratios in calculations

​Solution:

• where are we trying to go?

• what do we know?

• How do we get there?

​

How much O₂ will be produced by a reaction that begins with 5.8 moles of water (H₂O)?

​Example 2: using mole ratios in calculations

​Solution:

• where are we trying to go?

  • from 5.8 moles of H₂O to moles of O₂

• what do we know?

• How do we get there?

​

How much O₂ will be produced by a reaction that begins with 5.8 moles of water (H₂O)?

​Example 2: using mole ratios in calculations

​Solution:

• where are we trying to go?

  • from 5.8 moles of H₂O to moles of O₂

• what do we know?

  • Equation: 2H₂O --> 2H₂ + O₂

  • the ratio of H₂O to O₂ is 2:1

  • we have 5.8 moles of H₂O

• How do we get there?

​

How much O₂ will be produced by a reaction that begins with 5.8 moles of water (H₂O)?

​Example 2: using mole ratios in calculations

​Solution:

• where are we trying to go?

  • from 5.8 moles of H₂O to moles of O₂

• what do we know?

  • Equation: 2H₂O --> 2H₂ + O₂

  • the ratio of H₂O to O₂ is 2:1

  • we have 5.8 moles of H₂O

• How do we get there?

  • use the mole ratio as a CONVERSION FACTOR

​5.8 mol H₂O x 1 mol O₂ = 2.9 mol O₂

2 mol H₂O

Multiple Choice

How many moles of H₂O are needed to produce 3 moles of O₂? (hint: you may have to do some multiplying using a conversion factor)

Equation: 2H₂O → 2H₂ + O₂

• 2

• 4

• 6

• 8

Multiple Choice

Calculate the number of moles of oxygen that is required to react with 4.30 moles of propane, C₃H₈, in the reaction described by the following balanced chemical equation:

C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)

• 0.86 mol O₂

• 5 mol O₂

• 22.0 mol CO₂

• 21.5 mol O₂

​

Calculate the number of moles of oxygen that is required to react with 4.30 moles of propane, C₃H₈, in the reaction described by the following balanced chemical equation:

C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)

LET'S SOLVE IT TOGETHER...

​Solution:

• where are we trying to go?

  • 4.30 mol propane to moles O₂

• what do we know?

  • the ratio of propane to O₂ is 1:5

• How do we get there?

  • use the ratio as a conversion factor.

​4.30 mol C₃H₈ x 5 mol O₂ = 21.5 mol O₂

1 mol C₃H₈

​so it requires 21.5 moles of O₂