Write the equation sulfur dioxide reacts with oxygen to form sulfur trioxide.

2SO2 + 02 >2SO3

Explanation:

SO2 + O2 ===> SO3

The only thing you can do is balance the oxygens that are on the sulfurs. When you do that the oxygen (O2) can make the difference.

SO2 + O2 ===> 2SO3

Bring the oxygen on the right up to 6. When you do that, the sulfurs are out of balance. So balance the sulfurs.

2SO2 + 02 >2SO3

Your equation is now balanced. The trick is not to leave the number of oxygens on the right as an odd number. When you fix that problem, the equation will balance easily.

Species           S                 O

Left                  2                  2+4=6

Right                2                  2*3 =6

Balanced.