The usual procedure is to balance all atoms other than H and O; then balance O; then balance H.
Suppose you must balance the equation
CH₄ + O₂ → CO₂ + 2H₂O
Start with most complicated-looking molecule in the equation. This looks like CH₄.
1. Balance all atoms other than H and O.
Put a 1 in front of the CH₄, We start with
1 CH₄ + O₂ → CO₂ + 2H₂O
Since we have fixed one C atom on the left, we need one C atom on the right. We put a 1 in front of the CO₂..
1 CH₄ + O₂ → 1 CO₂ + H₂O
2. Balance O.
We can’t, because both O₂ and H₂O need coefficients. So we balance H instead.
3. Balance H.
Now we have fixed 4 H atoms on the left, so we need 4 H atoms on the right. We place a 2 in front of the H₂O.
1 CH₄ + O₂ → 1 CO₂ + 2 H₂O
4. Balance O.
We have four O atoms on the right, so we need four O atoms on the left. We put a 2 in front of the O₂.
1 CH₄ + 2 O₂ → 1 CO₂ + 2 H₂O
5. Check That the Equation is Balanced.
Since every formula has a coefficient, we should now have a balanced equation. Let’s check:
On the left: 1 C; 4 H; 4 O
On the right: 1 C; 4 O; 4 H.
Our balanced equation is
CH₄ + 2O₂ → CO₂ + 2H₂O