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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3?4
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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3?5
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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3?6
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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3?7
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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