Stoichiometry

Unit 6

Topic 1

• Intro / mole to mole

Stoichiometry

• Quantitative study of chemical reactions• The basic question - how much? - is the

object of stoichiometry.• Example: How many grams of silver

chloride are produced when 17.0 grams of silver nitrate react with excess sodium chloride?

Basic Stoichiometry• Always starts with a balanced equation.

• From the balanced equation we can determine the molar ratios.

• Remember that the coefficients refer to the number of moles of each reactant or product there are in the balanced equation.

• So in this equation 2 moles of aluminum oxide decompose to form 4 moles of aluminum metal and 3 moles of oxygen gas.

Al2O3 Al + O22 4 3

Basic Stoichiometry

• So for 2 moles of aluminum oxide, three moles of oxygen gas will be produced. Therefore, the molar ratio of aluminum oxide to oxygen gas would be 2:3

• What would the ratio of aluminum metal to oxygen gas be?

Al2O3 Al + O22 4 3

43

Writing Mole Factors

4 Fe + 3 O2 2 Fe2O3 Fe and O2 4 mole Fe and 3 mole O2

3 mole O2 4 mole Fe

Fe and Fe2O3 4 mole Fe and 2 mole Fe2O3

2 mole Fe2O3 4 mole Fe

O2 and Fe2O3 3 mole O2 and 2 mole Fe2O3

2 mole Fe2O3 3 mole O2

Learning Check S1

3 H2(g) + N2(g) 2 NH3(g)

A. A mole factor for H2 and N2 is1) 3 mole N2 2) 1 mole N2 3) 1 mole N2

1 mole H2 3 mole H2 2 mole H2

B. A mole factor for NH3 and H2 is1) 1 mole H2 2) 2 mole NH3 3) 3 mole N2

2 mole NH3 3 mole H2 2 mole NH3

1. Mole-to-Mole conversion

given Mole of A (coeff.)Mole of B (coeff.)Mole of A

Example4 Fe + 3 O2 2 Fe2O3

Given Unknown

How many moles of Fe2O3 are produced when

6.0 moles O2 react?

6.0 mole O2 2 mole Fe2O3 = 4.0 mole Fe2O3

3 mole O2

Practice 1

4 Fe + 3 O2 2 Fe2O3

How many moles of Fe are needed to react with 12.0 mole of O2?

Solution

4 Fe + 3 O2 2 Fe2O3

12.0 mole O2 4 mole Fe = 16.0 mole Fe 3 mole O2

Practice 2

4 Fe + 3 O2 2 Fe2O3

How many moles of O2 are needed to produce

0.400 mole of Fe2O3?

Topic 2

• Mass to mole or mole to mass

2. Mole to Mass/Mass to Mole conversion

mole of A (co) mole of B Molar mass of B (co) mole of A 1 mole of B

Mass(g) of A 1mole A (co)mole of B molar mass of A (co) mole of A

Example

4 Fe + 3 O2 2 Fe2O3

How many grams of O2 are needed to produce

0.400 mole of Fe2O3?

What’s given? Moles of Fe2O3

What’s unknown? Grams of O2

Plan: Convert moles of Fe2O3 to moles of O2. Then convert moles of O2 to grams of O2.

Solution

0.400 mole Fe2O3 3 mole O2 32.0 g O2

2 mole Fe2O3 1 mole O2

= 19.2 g O2

Practice

• 4 Fe + 3 O2 2 Fe2O3

• How many grams of Fe is needed to produce 0.400 mole of Fe2O3?

• How many grams of Fe2O3 is produced from 5 moles of Fe and excess O2?

Unit 6 Topic 3

• Mass to mass conversion

3. Mass to mass conversion

Mass(g) of A 1mole A (co)mole of B molar mass(g) of B molar mass(g) of A (co) mole of A 1mole B

The reaction between H2 and O2 produces 13.1 g of water. How many grams of O2 reacted?Write the equationH2 (g) + O2 (g) H2O (g)

Balance the equation 2 H2 (g) + O2 (g) 2 H2O (g)

Identify what’s given and what’s “unknown”

Example

2 H2 (g) + O2 (g) 2 H2O (g)

Start w/grams given

1 mole of givenMolar mass ofGiven(g)

Moles of“unknown”(coefficient)

Moles of given (coefficient)

Molar mass of“unknown”

1 mole“unknown”

grams “unknown”

=

given unknown

g H2O mole H2O mole O2 g O2

13.1 g H2O 1 mole H2O 1 mole O2 32.0 g O2 18.0 g H2O 2 mole H2O 1 mole O2

= 11.6 g O2

Practice

How many grams of O2 are needed to react w/ 50.0 grams of Na in the reaction

4 Na + O2 2 Na2O

Complete the set up:

50.0 g Na 1 mole Na 23.0 g Na

Solution

4 Na + O2 2 Na2O

50.0 g Na 1 mole Na 1 mole O2 32.0 g

23.0 g Na 4 mole Na 1 mole O2

= 17.4 g O2

Practice

Acetylene gas C2H2 burns in the oxyactylene torch for welding. How many grams of C2H2 are burned if the reaction produces 75.0 g of CO2?

2 C2H2 + 5 O2 4 CO2 + 2 H2O

Solution

2 C2H2 + 5 O2 4 CO2 + 2 H2O

75.0 g CO2 1 mole CO2 2 mole C2H2 26.0 g C2H2

44.0 g CO2 4 mole CO2 1 mole C2H2

= 22.2 g C2H2

Pathways for Problems Using Equations

Given (A) Unknown (B) grams (A) grams (B)

molar molar mass (A) mass (B) coefficients

moles (A) moles (B) Avogadro's Avogadro’s

number number

particles (A) particles (B)