Composition Stoichiometry

Mass Percentage mass of element in compound molar mass of compound x 100%

Percent Composition
The sum of the mass percents of all elements that make up the compound .
% A + %B
+ … + … =
100%

Determining Chemical Formulas

Mass Percentage mass of element in compound molar mass of compound x 100%

Percent Composition
The sum of the mass percents of all elements that make up the compound .
% A + %B
+ … + … =
100%

Determining Chemical Formulas

Empircal (Simplest) Formula
• The smallest whole-number ratio of atoms ( moles ) in the compound
• Calculated from composition data
• Empirical formula is accepted formula for ionic compounds

Determining Empirical Formula
• Convert composition data to masses in grams
– Assume
100g of substance ( if given percent composition)
• Convert masses in grams to amounts in moles
• Find the simplest whole-number ratio

CH
2
C
2
H
4
C
4
H
8
C
6
H
12
85.6% C, 14.4% H
85.6% C, 14.4% H
85.6% C, 14.4% H
85.6% C, 14.4% H

Calculating Molecular Formula
The molecular formula is proportional to the empirical formula
The molecular formula mass must be proportional to the empirical formula mass
Empirical Formula : Molecular Formula
Empirical Formula Mass : Molecular Mass

Calculating Molecular Formula
• Determine the empirical formula
• Calculate empirical formula mass
• Determine ratio of empirical formula mass to molecular mass
– molecular mass is experimentally determined
•
Apply the ratio to the simplest formula

Reaction Stoichiometry

Balanced Formula Equations
CH
4
(g) + O
2
(g)

CO
2
(g) + H
2
O (g)
1 atom C 1 atom C
4 atoms H
2 atoms O
2 atoms H
3 atoms O
4
4 3
2
(g)

CO
2
2
(g) + H
H
O (g)
2
O (g)

The Balanced Formula Equation
CH
4
(g) + 2 O
2
(g)

CO
2
(g) + 2 H
2
O (g)
•Establishes relationships among reactants and products
•Can compare two species at once
•Mole ratios
•Mass relationships derive from the mole relationships

Basic Stoichometric Relationships
• Mole-to-mole moles G
 moles W
• Mole-to mass moles G
 moles W
 mass W

Basic Stoichometric Relationships
• Mass-to-mole mass G
 moles G
 moles W
• Mass-to-mass mass G
 moles G
 moles W
 mass W

Basic Stoichometric Relationships
• Can convert from G

W only in moles
• Must use molar mass to convert
G or W into moles

Limiting Reagent and
Excess Reagent

Definitions
• Limiting Reagent
–Reactant that runs out first
–Causes reaction to stop
–Limits the amount of product
• Excess Reagent
–Reactant left over when reaction stops

How can I tell which one is the limiting reagent?

Finding the Limiting Reagent
* How much do I have?
* How much do I need?

Determining Limiting Reactant
“The Easy Way”
• Step 1: Find quantities of all reactants in moles
–convert from grams, if necessary

Determining Limiting Reactant
“The Easy Way”
• Step 2: Divide each reactant by its coefficient from the balanced equation
–Tells “how many” reactions can be fueled by that reactant
•Coefficient is moles per rxn

Determining Limiting Reactant
“The Easy Way”
• Step 3: The smallest quotient from Step 2 is LR
–can “fuel” the fewest number of reactions before it runs out

How can I tell how much of the excess reagent is left over?

Unreacted Excess Reactant
• Step 1: Determine LR
–as shown previously
• Step 2: Use given quantity of
LR as “starting point”
• Step 3: Calculate amount of ER needed to “use up” the LR
• Step 4:
Subtract amount needed from amount available

Theoretical Yield
Actual Yield
Per Cent Yield

2H
2
+ O
2

2H
2
O
• Theoretical Yield
– Maximum amount of product you can possibly get
–Based on the chemical equation
–Determined by the limiting reactant

Determining the Theoretical
Yield
• Find the limiting reactant
• Use the given amount of the limiting reactant to calculate product
–Do a regular stoichiometry problem

Per Cent Yield
• Compares actual yield to theoretical yield
• Measures the efficiency of the reaction
• Will usually be less than 100%

Actual Yield
• The measured amount of product actually obtained from the experiment
–Cannot be calculated from the chemical equation
–Must be determined in the lab
–Usually less than theoretical yield

% Yield =
Per Cent Yield x 100%

Empircal (Simplest) Formula
• The smallest whole-number ratio of atoms ( moles ) in the compound
• Calculated from composition data
• Empirical formula is accepted formula for ionic compounds

Determining Empirical Formula
• Convert composition data to masses in grams
– Assume
100g of substance ( if given percent composition)
• Convert masses in grams to amounts in moles
• Find the simplest whole-number ratio

CH
2
C
2
H
4
C
4
H
8
C
6
H
12
85.6% C, 14.4% H
85.6% C, 14.4% H
85.6% C, 14.4% H
85.6% C, 14.4% H

Calculating Molecular Formula
The molecular formula is proportional to the empirical formula
The molecular formula mass must be proportional to the empirical formula mass
Empirical Formula : Molecular Formula
Empirical Formula Mass : Molecular Mass

Calculating Molecular Formula
• Determine the empirical formula
• Calculate empirical formula mass
• Determine ratio of empirical formula mass to molecular mass
– molecular mass is experimentally determined
•
Apply the ratio to the simplest formula

Reaction Stoichiometry

Balanced Formula Equations
CH
4
(g) + O
2
(g)

CO
2
(g) + H
2
O (g)
1 atom C 1 atom C
4 atoms H
2 atoms O
2 atoms H
3 atoms O
4
4 3
2
(g)

CO
2
2
(g) + H
H
O (g)
2
O (g)

The Balanced Formula Equation
CH
4
(g) + 2 O
2
(g)

CO
2
(g) + 2 H
2
O (g)
•Establishes relationships among reactants and products
•Can compare two species at once
•Mole ratios
•Mass relationships derive from the mole relationships

Basic Stoichometric Relationships
• Mole-to-mole moles G
 moles W
• Mole-to mass moles G
 moles W
 mass W

Basic Stoichometric Relationships
• Mass-to-mole mass G
 moles G
 moles W
• Mass-to-mass mass G
 moles G
 moles W
 mass W

Basic Stoichometric Relationships
• Can convert from G

W only in moles
• Must use molar mass to convert
G or W into moles

Limiting Reagent and
Excess Reagent

Definitions
• Limiting Reagent
–Reactant that runs out first
–Causes reaction to stop
–Limits the amount of product
• Excess Reagent
–Reactant left over when reaction stops

How can I tell which one is the limiting reagent?

Finding the Limiting Reagent
* How much do I have?
* How much do I need?

Determining Limiting Reactant
“The Easy Way”
• Step 1: Find quantities of all reactants in moles
–convert from grams, if necessary

Determining Limiting Reactant
“The Easy Way”
• Step 2: Divide each reactant by its coefficient from the balanced equation
–Tells “how many” reactions can be fueled by that reactant
•Coefficient is moles per rxn

Determining Limiting Reactant
“The Easy Way”
• Step 3: The smallest quotient from Step 2 is LR
–can “fuel” the fewest number of reactions before it runs out

How can I tell how much of the excess reagent is left over?

Unreacted Excess Reactant
• Step 1: Determine LR
–as shown previously
• Step 2: Use given quantity of
LR as “starting point”
• Step 3: Calculate amount of ER needed to “use up” the LR
• Step 4:
Subtract amount needed from amount available

Theoretical Yield
Actual Yield
Per Cent Yield

2H
2
+ O
2

2H
2
O
• Theoretical Yield
– Maximum amount of product you can possibly get
–Based on the chemical equation
–Determined by the limiting reactant

Determining the Theoretical
Yield
• Find the limiting reactant
• Use the given amount of the limiting reactant to calculate product
–Do a regular stoichiometry problem

Per Cent Yield
• Compares actual yield to theoretical yield
• Measures the efficiency of the reaction
• Will usually be less than 100%

Actual Yield
• The measured amount of product actually obtained from the experiment
–Cannot be calculated from the chemical equation
–Must be determined in the lab
–Usually less than theoretical yield

% Yield =
Per Cent Yield x 100%