Question

Balance the following equations.
A) SO2 + O2 --> SO3
B)Fe2O3 + H2 --> Fe +H2O
C)P + O2 -->P4O10
D)Al + N2 --> AlN

Answer 1

This is done by trial and error. But I will go through the first one.
SO2 + O2 ==> SO3

1. Take them in order.
2. S balances so we are ok.
3. O doesn't balance AND I notice that we have an even/odd number of the two sides so we KNOW we can never balance them unless we use something like the common denominator for 2 and 3. That would be 6 so we do it this way.
S + 3O2 ==> 2SO3 which takes care of the O nicely. Of course, it messes up the S BUT we can fix that.
4. We add a 2 for the reactant S.
2S + 3O2 ==>2SO3

THEN we ALWAYS check it at the end.
I see 2 S on each side.
I see 6 O on each side.
DONE.

Practice makes perfect. As you do more of these you gain experience which helps tell you where to start balancing, which ones to look for trick of the trade, etc. Good luck.