Balancing chemical equationsWatch
That's really all there is to it.
As an example:
C4H10 + O2 -> CO2 + H2O
As you can see, there is 4 C on the left but only 1 on the right. 10 H on the left but only 2 on the right. 2 O on the left but 3 on the right. Not balanced.
First balance C
C4H10 + O2 -> 4 CO2 + H2O
C4H10 + O2 -> 4 CO2 + 5 H2O
Finally need to do O and it gets a bit tricky. We have 8 O in the 4 CO2 and 5 O in the 5 H2O. 8 + 5 = 13 so we need to get 13 O atoms on the left. Since it is O2 if we do 13/2 (or 6.5) O2 then it will balance.
C4H10 + 13/2 O2 -> 4 CO2 + 5 H2O
But we don't really like seeing that fraction, so instead if we multiply the entire equation by 2 it will get rid of it.
2 C4H10 + 13 O2 -> 8 CO2 + 10 H2O
Before you balance the sides of the equation you must make sure each formula you are using is the correct formula. Don't try to manipulate formulae for convenience - the formula is the formula regardless of how easy it makes balancing the equation.
It sounds obvious, but I have come across so many students who try to change formulae to make the balancing easier.
You may also want to have a look at my post here
Good luck with your test.