Unit 6 Stoichiometry

What Exactly Is Stoichiometry?

Composition stoich – deals with mass relationships of elements in compounds (review Ch 3)

Reaction stoich – deals with relationships between reactants and products

What You’ve Learned So Far…

•Unit Conversions•Naming Chemical Compounds•Writing Chemical Formulas •The Mole•Writing Chemical Equations •Balancing Chemical Equations

Mole Ratios•Conversion factor relating amount of moles of any two substances

Example(g)3O 4Al(s) (aq)O2Al 232

Al 4

OAl 2 32

32OAl 2

Al 4

Al 4

O 3 2

2

32

O 3

OAl 2

32

2

OAl 2

O 3

2O 3

Al 4

or

or

or

Molar Mass Ratios

Al2O3 – 101.96 g/mol

Al – 26.98 g/mol

O2 – 32.00 g/mol2

2

2

2

32

32

32

32

O g 00.32

O mol 1or

O mol 1

O g 00.32

Al g 26.98

Al mol 1or

Al mol 1

Al g98.26

OAl g 96.101

OAl mol 1or

OAl mol 1

OAl g 96.101

(g)3O 4Al(s) (aq)O2Al 232

Solving Stoichiometry

Problems

Must have:1. Correct molecular

formulas2. Balanced chemical

equation

Steps to Solving Problems

1. Write a balanced chemical equation

2. Identify information given in problem

3. Pick appropriate molar mass or mole ratio

4. Multiply numerators5. Multiply denominators

Reaction Stoichiometry

Mole-MoleMole-Mass or Mass-Mole

Mass-Mass

Type 1

• Given quantity and unknown quantity in moles

Quantity givengiven (in mol)

Quantity unknownunknown (in mol)

Stoichiometry Examples: Type 1 (mol-mol)Example 1: The combustion of 2.19

moles propane (C3H8) produces how many moles carbon dioxide?

Example 2: 41.8 mol lithium hydroxide is combined with carbon dioxide to produce lithium carbonate and liquid water. How much water is produced from this reaction?

Type 2

• Given amount is in moles and unknown is mass in grams

Amount givengiven (in mol)

amount unknownunknown (in mol)

mass unknownunknown (in g)

Stoichiometry Examples: Type 2 (mol-mass)

Example 3:In photosynthesis, plants use energy

from the sun to produce glucose C6H12O6, and oxygen from the reaction of carbon dioxide and water. What mass, in grams, of glucose is produced when 3.00 mol of water react with carbon dioxide?

Type 3

• Given is amount in grams and unknown is amount in moles

mass givengiven (in g) amount givengiven (in mol)

amount unknownunknown (in mol)

Stoichiometry Examples: Type 3 (mass-mol)NaHCO3 and Mg(OH)2 are both used as

antacids – which is more effective per gram?

Example 4: 1.00 g NaHCO3 reacts with hydrochloric acid to produce sodium chloride, water, and carbon dioxide

Example 5: 1.00 g Mg(OH)2 reacts with hydrochloric acid to produce magnesium chloride and water

Stoichiometry Examples:Type 3 (mol-mass)

Example 6What mass of carbon dioxide, in

grams, is needed to react with 3.50 mol of water in the photosynthetic reaction?

ObjectiveIII.A.3(i) – Use chemical equations

to perform basic mole-mole, mass-mass, and mass-mole computations for chemical reactions

Type 4

• Given is mass in grams and unknown is mass in grams

mass givengiven (in g) amount givengiven (in mol)

amount unknownunknown (in mol) mass unknownunknown (in

g)

Stoichiometry Examples:Type 4 (mass-mass)

Example 7Tin (II) fluoride is used in some

toothpastes. It is made by the reaction of tin with hydrogen fluoride according to the following equation.

Sn(s) + 2HF SnF2(s) + H2(g)

How many grams of SnF2 are produced from the reaction of 30.00 g of HF with Sn?

Limiting Reagents

III.A.3(j) – identify limiting reagents and use this information when solving stoichiometry problems

What is a “limiting reagent”?

• The reactant that is completely used up first in a chemical reaction

How to Find the Limiting Reagent

1. Analyze the question2. Identify the given

information3. Write a balanced chemical

reaction4. Convert given information of

Reactant 1 to Product A5. Convert given information of

Reactant 2 to Product A

Theoretical Yield

III.A.3(k) – compute theoretical yield, actual (experimental) yield, and percent yield

III.A.3(l) – calculate percent error and analyze experimental errors that affect

percent error

Theoretical Yield

Using the limiting reagent to calculate the amount of product produced is called the theoretical yield

This is how much product could be made if the limiting reactant was totally consumed

Experimental Yield

Also called the actual yield

The amount of product actually collected in the laboratory is called the experimental yield

Percent Yield

yieldpercent 100yield ltheoretica

yield actual

Example – Limiting ReactantAluminum reacts with chlorine gas to

form aluminum chloride.

In a certain experiment, 10.0 g of aluminum is reacted with 35.0 g of chlorine gas.

What mass of aluminum chloride will be produced, assuming a complete reaction?

What mass of which reactant is left after the reaction?