Bell Work: Mole Ratios

2 C4H10  + ___ O2  →   ___ CO2  + ___ H2O

1. Finish balancing the equation.How many moles of . . . . . .

2. oxygen will react?

3. water will be produced?

4. butane will react?

5. carbon dioxide will be produced?

Stoichiometric Calculations

Section 12.2

What is Stoichiometry?

The study of the relationship between amounts of reactants used and products formed in a chemical reaction

Based on the Law of Conservation of Mass and Matter Matter is neither created nor destroyed Mass of reactants equals the mass of the

products

PROBLEM Interpret in terms of particles, moles, and mass.

Show that mass is conserved:

(Hint: look at coefficients for particles & moles)

4 Al + 3O2 2 Al2O3

Particles:

Moles:

Mass:

Conserved?

SOLUTION: 4Al + 3O2 2 Al2O3

Particles: 4 molecules 3 molecule 2 molecule

Moles: 4 mole 3 mole 2 moles

• Mass: 4 (27.0 g) + 3 (26.0 g) = 2 (102.0 g)

• Conserved? 204.0 g = 204.0 g YES!

• Law of Conservation of Matter shown.

Conservation of MassDoes the equation have conservation of mass?

4Fe (s) + 3O2 (g) 2Fe2O3 (s)

4Fe 3O2 2Fe2O3

Molecules

Moles

Grams

4Fe 3O2 2Fe2O3

Molecules 4 3 2

Moles

Grams

4Fe 3O2 2Fe2O3

Molecules 4 3 2

Moles 4 3 2

Grams

4Fe 3O2 2Fe2O3

Molecules 4 3 2

Moles 4 3 2

Grams 4(55.8 g Fe) = 223.2 g Fe

3(32.0 g O2) = 96.0 g O2

2(159.6 g Fe2O3)= 319.2 g Fe2O3

YES!

Using Stoichiometry

In order to use stoichiometry correctly, you must start with a balanced chemical equation!

The mole ratios needed come from the coefficients in a balanced chemical equation.

First we will examine mole-to-mole conversions.

MOLE RATIO – is a ratio between the number of moles of any two substances in a balanced chemical equation There are three mole ratio pairs for the

following:

Ex. 4 Al(s) + 3 O2(g) 2 Al2O3(s)

Note: 4 moles 3 moles 2 moles

4 mol Al and 3mol O2

3 mol O2 4 mol Al

MOLE RATIO – is a ratio between the number of moles of any two substances in a balanced chemical equation 4 mol Al and 2 mol Al2O3

2 mol Al2O3 4 mol Al

3 mol O2 and 2 mol Al2O3

2 mol Al2O3 3 mol O2

All stoichiometry calculations begin with a balanced equation and mole ratios!!

Using Stoichiometry

Mole ratios can be used to convert the known number of moles of a substance to an unknown.

Remember that when you use a conversion factor, the units must cancel out.

moles of unknown

moles of known x = moles of unknownmoles of known

Mole-to-Mole Conversions

Example: Sulfuric acid is formed when sulfur dioxide reacts with oxygen and water. If 12.5 mol sulfur dioxide reacts, how many mol sulfuric acid can be produced? How many mol oxygen are needed?

First, write the balanced chemical equation.

SO2 + H2O + O2 H2SO4 Is this balanced?

No… 2SO2 + 2H2O + O2 2H2SO4

Mole-to-Mole Conversions

2SO2 + 2H2O + O2 2H2SO4

How much sulfuric acid is produced if 12.5 mol sulfur dioxide reacts?

Start with what you are given.

12.5 mol SO2 x (mole-to-mole ratio)

2 4

2 2 42

2 mol H SO12.5 mol SO  x 12.5 mol H SO

2 mol SO

Mole-to-Mole Conversions

2SO2 + 2H2O + O2 2H2SO4

How much oxygen in needed if 12.5 mol sulfur dioxide reacts?

Start with what you are given.

12.5 mol SO2 x (mole-to-mole ratio)

2

2 22

1 mol O12.5 mol SO  x 6.25 mol O

2 mol SO

Mole-to-Mass Conversions

Example:

Determine the mass of sodium chloride produced when 1.25 moles of chlorine gas reacts vigorously with sodium.

First, write a balanced chemical equation: 2Na (s) + Cl2 (g) 2NaCl (s)

Example

2Na (s) + Cl2 (g) 2NaCl (s) Write what is given. Use the mole ratio that relates mol NaCl to mol Cl2.

2

2 mol NaCl

1 mol Cl

22

2 mol NaCl 58.44 g NaCl1.25 mol Cl x x 146 g NaCl

1 mol Cl 1mol NaCl

Determine the molar mass of NaCl.Molar mass NaCl = 58.44 g/mol

Mass-to-Mass ConversionsExample:

Determine the mass of potassium chlorate needed to produce 100.0 grams of oxygen.

Write a balanced chemical equation:

2 KClO3 (s) 2 KClO2 (s) + O2 (g)

Example 2 KClO3 (s) 2 KClO2 (s) + O2 (g)

Use the mole ratio that relates mol O2 to mol KClO3.

1 mol O2 / 2 mol KClO3

Determine the molar mass of KClO3 and O2. MM KClO3 = 122.6 g/mol and MM O2 = 32.0 g/mol

Start with what you are given.

33

3

2

3

2

2 2 KClO g 766.3

KClO mol 1

KClO g 122.6x

O mol 1

KClO mol 2x

O g 32.0

O mol 1x O g 100.0

Practice Problem (mass-to-mass)

Pg. 362 #13

Steps in Stoichiometric Calculations

1) Write a balanced chemical equation, and interpret the equation in terms of moles.

2) Determine the moles of the given substance using a mass-to-mole conversion.

3) Determine the moles of the unknown substance from the moles of the substance given (mole-to-mole conversion).

4) From the moles of the unknown substance, determine the mass of the unknown substance using a mole-to-mass conversion.