I-6. 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: C₈H_{18 (l)} + O_{2 (g)} ?

The products of this reaction are CO_{2 (g)} and H₂O_(l)

The unbalanced reaction is:
C₈H_{18 (l)} + O_{2 (g)} CO_{2 (g)} + H₂O_(l)

Start balancing this reaction with carbon. There are 8 carbons on the left so we need 8 carbons on the right:
C₈H_{18 (l)} + O_{2 (g)} 8 CO_{2 (g)} + H₂O_(l)

Next balance the hydrogens. There are 18 hydrogens on the left so we need 18 hydrogens on the right:
C₈H_{18 (l)} + O_{2 (g)} 8 CO_{2 (g)} + 9 H₂O_(l)

Finally balance the oxygens. There are 25 oxygens on the right so we need 25 oxygens on the left:
C₈H_{18 (l)} + 12.5 O_{2 (g)} 8 CO_{2 (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

• The reaction of H₂ + O₂