Molar Ratios­Notes.notebook
December 17, 2012
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Finding and Using Molar Ratios
The coefficients in a balanced chemical equation can be
used to determine the relative number of
molecules, formula units, or moles of a compound
involved in a chemical reaction.
Steps:
1) Balance equation
2) Set up BUA (before, used, after)
3) Divide by own, multiply by where going
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Molar Ratios­Notes.notebook
December 17, 2012
Example:
Example: For the following reaction use the equation given to
determine the number of moles of Li3N that can be prepared
using 0.24 mol Li.
6 Li (s) +
N 2 (g) --> 2 Li3N (s)
B
.24
.04
-
U
.24
.04
-
A
-
-
0.08
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Molar Ratios­Notes.notebook
December 17, 2012
How many moles of water will be produced from the
combustion of 0.27 moles of CH3OH?
2 CH3OH (l) + 3 O2 (g) --> 2 CO2 (g) + 4 H2O (l)
B
U
A
0.27
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Molar Ratios­Notes.notebook
December 17, 2012
How many moles of O 2 are needed to burn 2.56
moles of CH 3OH?
2 CH3OH (l) + 3 O2 (g) --> 2 CO2 (g) + 4 H2O (l)
B
2.56
U
A
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--
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Molar Ratios­Notes.notebook
December 17, 2012
Can you determine the number of moles of hydrogen
produced when 0.0400 mole of potassium reacts with
water?
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Molar Ratios­Notes.notebook
December 17, 2012
If 14.0 moles of butane (C 4H10 ) is burned,
how many moles of O 2 are required?
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Molar Ratios­Notes.notebook
December 17, 2012
Limiting Reactants
The limiting reactant limits the extent of the
reaction and thereby determines the amount of the
product. A portion of all of the other reactants
remains after the reaction stops. These left over
reactants are called excess reactants.
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Molar Ratios­Notes.notebook
December 17, 2012
Example:
If 200.0g of sulphur reacts with 100.0g of chlorine, what
mass of disulfur diochloride is produced?
100.0g Cl2 = 1.410 mol Cl2
200.0g S8 = 0.7797 mol S8
B
U
A
S8(l) +
0.7797
0.3525
0.4272
4Cl2(g)
1.410
1.410
0
à
4S2Cl2(l)
1.410
1.410mol S2Cl2 x 134.9g/mol = 190.1g S2Cl2
Cl2 is the limiting reagent.
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Molar Ratios­Notes.notebook
December 17, 2012
The reaction between solid phosphorous and oxygen produces
solid tetraphosphorous decoxide. Determine the mass of
teraphosphorous decoxide formed if 25.0g of phosphorous (P4)
and 50.0 g of oxygen are combined.
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