HOMEWORK HELP | This law states that matter cannot be created or destroyed.

Virtual Lab – Chemical Equations

This law states that matter cannot be created or destroyed. This means that during a chemical reaction there are the same number of atoms before and after. However, not all molecules react in a one to one ratio.

Mg  +  O₂  →  MgO

Because oxygen is a diatomic molecule (O
₂
) in a one to one ratio, there is an oxygen atom left over that must be used. If we add another atom of magnesium it can bond with that leftover oxygen to produce two molecules of MgO.

2Mg  +  O₂
→  2MgO

This equation is now balanced, meaning that it shows the correct ratio so that there are the same number of atoms before and after the reaction.

When balancing a chemical equation you may NOT change any chemical formulas or add any subscripts.

1. Count the number of each type of atom on each side of the arrow.
2. If a polyatomic ion is on both sides of the reaction, you may count it as one ion.
3. Add coefficients in front of chemical formulas in order to balance the equation.
4. Balance hydrogen and oxygen (if present) last.
5. Make sure your have the lowest ratio possible when done. (Reduce all coefficients if possible.)

Watch the video below to watch me work through these two examples, plus 1 more. You might want to pause the video at times so that you can practice and check your work.

Example 1: Balance the following equation.

___CH₄  +  ___O₂
→  ___CO₂  +  ___H₂O

              C – 1                                   C – 1

              H – 4                                   H – 2

              O – 2                                   O – 3
(2 + 1)

We can see that our carbons are balanced, but our hydrogen and oxygen atoms are both unbalanced. We may only add coefficients in the underlined spaces. In order to make there be 4 hydrogen on the right side of the equation we should add a 2 in front of H
₂
O.

___CH₄  +  ___O₂
→  ___CO₂  +  _2_H₂O

This coefficient balances our hydrogen atoms. It also changes how many oxygen atoms we have on the right side of our equation, so we change our count. Now in order to balance our oxygen atoms we should put a 2 in front of the O
₂
.

___CH₄  +  _2_O₂
→  ___CO₂  +  _2_H₂O

O –
2
4                               O –
3
4

Balancing is a process of trial and error and you will only get better at it with practice. Also, there are sometimes many paths to the same correct answer. If you find yourself going in circles while balancing a problem, start fresh and don’t get discouraged.

Example 2:  Balance the following equation.

___K₃PO₄  +  ___HCl  →  ___KCl  +  ___H₃PO₄

K – 3

K – 1

              PO
₄
– 1                       PO
₄
– 1
We can count this as ONE item because PO₄ stays together.

                   H – 1                           H – 3

                  Cl – 1                           Cl – 1

We can see that the K and H are not balanced. We can balance the potassium by putting a 3 in front of KCl.

Now we can balance our hydrogen by placing a 3 in front of HCl.

Cl –
1
3                       Cl –
1
3

Click HERE for some more balancing practice. This is not for a grade but will help you gain confidence in your balancing skills!