3.9 Stoichiometric Calcs: Amounts of Reactants and Products

[Pages:24]3.9 Stoichiometric Calcs: Amounts of

Reactants and Products

Law of Conservation of Mass

The Law of Conservation of Mass indicates that in an ordinary chemical reaction,

? Matter cannot be created nor destroyed. ? No change in total mass occurs in a reaction. ? Mass of products is equal to mass of reactants.

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3?2

Conservation of Mass

2 moles Ag + 1 moles S 2 (107.9 g) + 1(32.1 g)

247.9 g reactants

= 1 mole Ag2S = 1 (247.9 g) = 247.9 g product

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3?3

Reading Equations In Moles

Consider the following equation:

4 Fe(s)

+ 3 O2(g)

2 Fe2O3(s)

This equation can be read in "moles" by placing the word "moles" between each coefficient and formula.

4 moles Fe + 3 moles O2

2 moles Fe2O3

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Writing Mole-Mole Factors

A mole-mole factor is a ratio of the moles for any two

substances in an equation.

4Fe(s) + 3O2(g)

2Fe2O3(s)

Fe and O2 Fe and Fe2O3 O2 and Fe2O3

4 moles Fe and 3 moles O2

3 moles O2

4 moles Fe

4 moles Fe and 2 moles Fe2O3

2 moles Fe2O3

4 moles Fe

3 moles O2 and 2 moles Fe2O3

2 moles Fe2O3

3 moles O2

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Learning Check

Consider the following equation:

3 H2(g) + N2(g)

2 NH3(g)

A. A mole-mole factor for H2 and N2 is

1) 3 moles N2 2) 1 mole N2

1 mole H2

3 moles H2

3) 1 mole N2 2 moles H2

B. A mole-mole factor for NH3 and H2 is

1) 1 mole H2 2) 2 moles NH3 3) 3 moles N2

2 moles NH3 3 moles H2

2 moles NH3

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Solution

3H2(g) + N2(g)

2NH3(g)

A. A mole-mole factor for H2 and N2 is 2) 1 mole N2

3 moles H2

B. A mole-mole factor for NH3 and H2 is 2) 2 moles NH3 3 moles H2

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Calculations with Mole Factors

How many moles of Fe2O3 can form from 6.0 mole O2?

4Fe(s) + 3O2(g)

2Fe2O3(s)

Relationship:

3 mole O2 = 2 mole Fe2O3

Write a mole-mole factor to determine the moles of Fe2O3.

6.0 mole O2 x 2 mole Fe2O3 = 4.0 mole Fe2O3 3 mole O2

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3?8

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