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Stoichiometry

Introduction

Atoms combine in integral ratios to form stable molecules. The number of each atom must therefore be equal on each side of a reaction. This requirement is called mass balance. The other property that must be equal is the total charge on each side of a reaction. This requirement is called charge balance.


Example 1

Combustion of octane: C8H18 (l) + O2 (g) ---> ?

The products of this reaction are CO2 (g) and H2O(l)

The unbalanced reaction is:
C8H18 (l) + O2 (g) ---> CO2 (g) + H2O(l)

Start balancing this reaction with carbon. There are 8 carbons on the left so we need 8 carbons on the right:
C8H18 (l) + O2 (g) ---> 8 CO2 (g) + H2O(l)

Next balance the hydrogens. There are 18 hydrogens on the left so we need 18 hydrogens on the right:
C8H18 (l) + O2 (g) ---> 8 CO2 (g) + 9 H2O(l)

Finally balance the oxygens. There are 25 oxygens on the right so we need 25 oxygens on the left:
C8H18 (l) + 12.5 O2 (g) ---> 8 CO2 (g) + 9 H2O(l)

The reaction is now mass balanced. The charge on each side of the reaction is 0, so the reaction is also charge balanced and we are done.


Example 2

Neutralization of an acidic solution of iron: Fe3+(aq) + OH-(aq) ---> ?

The product of this reaction is Fe(OH)3 (s).

The unbalanced reaction is:
Fe3+(aq) + OH-(aq) ---> Fe(OH)3 (s)

There is one Fe on each side of the reaction, so now balance the hydroxide. There are 3 hydroxides on the right so we need 3 hydroxides on the left:
Fe3+(aq) + 3 OH-(aq) ---> Fe(OH)3 (s)

The reaction is now mass balanced. The charge on the left side of the reaction is +3 + 3(1-) = 0, and the charge on the right side of the reaction is 0 so the reaction is also charge balanced and we are done.


Practice Reactions to Balance


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