STOICHIOMETRY - cdlilly.com 12 Lecture.pdfSTOICHIOMETRY Chemists use balanced chemical equations to...

2/25/2014

STOICHIOMETRY

Stoikheion = element; metron = to measure

STOICHIOMETRY Chemists use balanced chemical equations to

calculate how much reactant is needed or how

much product is formed.

– provides the same kind of quantitative

information that a recipe does.

STOICHIOMETRY is…. • Calculation of the quantities of reactants

and products in a chemical reaction.

• The study of the quantitative relationships

between the amounts of reactants and

products in a chemical reaction.

• Based on the Law of Conservation of Mass

– MASS and the NUMBER OF ATOMS are

conserved in every chemical reaction.

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Interpreting Chemical

Equations • A balanced chemical equation can be

interpreted in terms of different quantities,

including numbers of atoms, molecules, or

moles; mass; and volume.

Equations

Number of Atoms

Equations

Number of Molecules

1 molecule N2 + 3 molecules H2 2 molecules NH3

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Equations

Number of Molecules

Equations

1 mole N2 + 3 moles H2 2 moles NH3

Equations

28 g N2 + 3 x 2 g H2 2 x 17 g NH3

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Equations

28 g N2 + 3 x 2 g H2 2 x 17 g NH3

34 g reactants 34 g products

Equations

Volume

28 g N2 + 3 x 2 g H2 2 x 17 g NH3

34 g reactants 34 g products

Mole Ratios in Chemical Reactions

• The ratio between the coefficients of any two

compounds in a balanced chemical reaction.

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

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Stoichiometry Vocabulary • Stoikheion = element; metron = to measure

• Study of the quantitative relationships in

chemical formulas & equations

• Review: a mole (mol) is a certain number of things

(atoms, ions or molecules) = to 6.02 x 1023

• Coefficients in the balanced equation tell you how

many moles of each reactant and product are needed

Stoichiometry Vocabulary

• Review: formula or molar mass is found by

adding the atomic masses on the periodic

table (round to the tenths place)

• Molar volume (measured at STP “standard

temperature & pressure”) of any gas is equal

to 22.4 L

Chapter 11 v. Chapter 12 In Chapter 11, we converted amounts of the

SAME substance to different units (moles,

particles, L or g).

In Chapter 12, we will be looking

at chemical equations and

calculating amounts of a

substance using a given amount

of a DIFFERENT substance.

Mole ratios are the

HEART of every

stoichiometry problem.

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Mole Map

Volume Volume

Given Wanted

Particles Moles Moles Particles

Given Given Wanted Wanted

Mass Mass

Given Wanted

Mole Map

Mass Moles Moles Mass

Mole-Mole Conversions

Mole ratios are the HEART of every

stoichiometry problem.

Given (moles of the

known) # moles unknown

= calculated moles of unknown

# moles known

MOLE RATIO USING

COEFFICIENTS FROM CHEM.

The mole ratio you use depends on what

substance your given is (“known”) and which

substance you want to calculate (“unknown”)

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• Problem: If I have 2 moles of N2 and excess

H2, how many moles of NH3 can I make?

“KNOWN” (given):

“UNKNOWN”:

MOLE RATIO:

Mole-Mole Conversions - Example

1 N2 + 3 H2 2 NH3

2 moles N2

moles NH3

N2 + 3 H2 2 NH3

Given (moles of the

# moles known

MOLE RATIO USING

2 moles N2 2 moles NH3 = 4 moles NH3

1 mole N2

• Problem: If I have 8 moles of O2 and excess

H2, how many moles of H2O can I make?

“KNOWN” (given):

“UNKNOWN”:

MOLE RATIO:

2 H2 + 1 O2 2 H2O

8 moles O2

moles H2O

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2 H2 + O2 2 H2O

Given (moles of the

# moles known

MOLE RATIO USING

8 moles O2 2 moles H2O = 16 moles H2O

1 mole O2

Mole-Mole Calculations – You try…

4 Al + 3 O2 2 Al2O3

1. KNOWN: UNKNOWN:

12 moles Al2O3 moles O2

2. Mole ratio:

3. Calculate unknown:

OAl moles 2

O moles 3

known moles

unknown moles

2O moles 18 32

OAl moles 2

O moles 3OAl moles 12

•If I have 12 moles of Al2O3 how many moles of O2 do I need?

Mole Map

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Mole-Mass Conversions

Conversion if you know the number

of moles of a reactant or product in

a reaction and you want to calculate

the mass of another product or

reactant.

Mole-Mass Calculations - Example

Problem:

• If I have 2 moles of N2 and excess H2, how many grams

of NH3 can I make?

N2 + 3 H2 2 NH3

Given (moles of

the known) # moles unknown g unknown = calculated

mass (g) of

unknown # moles known 1 mole unknown

MOLE RATIO USING

MOLAR MASS OF UNKNOWN

2 moles N2 2 moles NH3 17.0 g NH3 = 68.0 g NH3

1 mole N2 1 mole NH3

Mole-Mass Calculations - Example

Problem:

• If I have 8 moles of O2 and excess H2, how many grams

of H2O can I make?

2 H2 + O2 2 H2O

Given (moles of

the known) # moles unknown g unknown = calculated

mass (g) of

unknown # moles known 1 mole unknown

MOLE RATIO USING

MOLAR MASS OF UNKNOWN

8 moles O2 2 moles H2O 18.0 g H2O

= 288.0 g H2O 1 mole O2

1 mole H2O

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Mass-Mole Calculation - Example

Problem:

• If I have 8 grams of O2 and excess H2, how many moles

of H2O can I make?

2 H2 + O2 2 H2O

Given (grams of

the known) 1 mole known # moles unknown = calculated

moles of

unknown grams known # moles known

MOLAR MASS OF KNOWN MOLE RATIO USING

8 g O2 1 mole O2 2 moles H2O = 0.5 mole

H2O 32.0 g O2

1 mole O2

Mole Map

Mass-Mass Conversions Conversion if you know the mass of a reactant or

product in a reaction and you want to calculate the

mass of another product or reactant.

(grams

known) 1 mole known # moles unknown grams unknown = calculated

mass (g) of

unknown grams known # moles known 1 mole unknown

MOLAR MASS OF

MOLE RATIO USING

COEFFICIENTS FROM

CHEM. RXN.

MOLAR MASS OF

UNKNOWN

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Mass-Mass Calculations - Example

Problem:

• If I have 2.0 grams of N2 and excess H2, how many grams

of NH3 can I make?

N2 + 3 H2 2 NH3

(grams

known) 1 mole known # moles unknown grams unknown

= calculated

mass (g) of

MOLAR MASS OF

MOLE RATIO USING

COEFFICIENTS FROM

CHEM. RXN.

MOLAR MASS OF

UNKNOWN

2.0 g N2 1 mole N2 2 moles NH3 17.0 grams NH3

= 2.4 g NH3

28.0 grams N2 1 moles N2 1 mole NH3

Problem:

• If I have 37.0 grams of H2, how many grams of O2 will

react?

2 H2 + O2 2 H2O

(grams

= calculated

mass (g) of

MOLAR MASS OF

MOLE RATIO USING

COEFFICIENTS FROM

CHEM. RXN.

MOLAR MASS OF

UNKNOWN

37.0 g H2 1 mole H2 1 moles O2 32.0 grams O2

= 296 g O2

2.0 grams H2 2 moles H2 1 mole O2

Problem:

react?

2 H2 + O2 2 H2O

(grams

= calculated

mass (g) of

MOLAR MASS OF

MOLE RATIO USING

COEFFICIENTS FROM

CHEM. RXN.

MOLAR MASS OF

UNKNOWN

37.0 g H2 1 mole H2 1 moles O2 32.0 grams O2

= 296 g O2

2.0 grams H2 2 moles H2 1 mole O2

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Mole Map

Mixed Stoichiometry Calculations

Problem:

• If I have 37.0 grams of H2, how many moles of O2 will

react?

2 H2 + O2 2 H2O

(grams

known) 1 mole known # moles unknown = calculated

moles of

unknown grams known # moles known

MOLAR MASS OF

MOLE RATIO USING

COEFFICIENTS FROM

CHEM. RXN.

Problem:

• If I have 2.0 moles of H2, how many grams of O2 will

react?

2 H2 + O2 2 H2O

Given (moles

unknown = calculated

mass (g) of

unknown # moles known

1 mole

unknown

MOLE RATIO USING

COEFFICIENTS FROM

CHEM. RXN.

MOLAR MASS OF

UNKNOWN

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Problem:

• If I have 2.0 moles of H2, how many moles of O2 will

react?

2 H2 + O2 2 H2O

Given (moles

known) # moles unknown = calculated

moles of

unknown # moles known

MOLE RATIO USING

Problem:

react?

2 H2 + O2 2 H2O

(grams

= calculated

mass (g) of

MOLAR MASS OF

MOLE RATIO USING

COEFFICIENTS FROM

CHEM. RXN.

MOLAR MASS OF

UNKNOWN

STOICHIOMETRY - cdlilly.com 12 Lecture.pdfSTOICHIOMETRY Chemists use balanced chemical equations to...

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