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Examples of Balancing Chemical Equations

This is a set of worked examples of how to balance chemical equations. Try each question first, using a pen and paper. Then click the word Answer and magically all will be revealed, without even leaving this page. Click anywhere else on the page to close the answer and move on to the next question. If the answer is long, scroll down until the example is at the top of the screen before clicking it.

Example 1

C5H12 + O2 ---> CO2 + H2O

Answer » There are five carbons on the left but only one on the right, and on each side the carbon is in a single chemical species. Put a 5 in front of the CO2 on the right hand side.

C5H12 + O2 ---> 5CO2 + H2O

There are twelve hydrogens on the left but only two on the right hand side, and hydrogen is in a single species on each side. Put a 6 in front of the H2O on the right hand side.

C5H12 + O2 ---> 5CO2 + 6H2O

Finally, there are only two oxygens on the left hand side but 16 of them on the right hand side. So put a 8 in front of the H2O on the right hand side.

C5H12 + 8O2 ---> 5CO2 + 6H2O
It's now a balanced chemical equation.

Example 2

Zn + HCl ---> ZnCl2 + H2

Answer » There are two chlorines on the right but only one on the left, and the chlorine is in a single chemical species on each side . Put a 2 in front of the HCl on the left hand side.

Zn + 2HCl ---> ZnCl2 + H2

And if you look carefully, you will see that the equation is now balanced, with one Zn on each side, two hydrogens on each side and two chlorines on each side. Some examples can be rather easy!

Example 3

Ca(OH)2 + H3PO4 ---> Ca3(PO4)2 + H2O

Answer » There are three calciums on the right but only one on the left, and the calcium is in a single chemical species on each side . Put a 3 in front of the Ca(OH)2 on the left hand side.

3Ca(OH)2 + H3PO4 ---> Ca3(PO4)2 + H2O

There are two PO4 ions on the right but only one on the left side, and the P doesn't appear anywhere else (so the group remains intact). Put a 2 in front of the H3PO4 on the left side.

3Ca(OH)2 + 2H3PO4 ---> Ca3(PO4)2 + H2O

Finally, there are six oxygens on the left hand side not present as PO4 but only one on the right hand side not in the PO4. So put a 6 in front of the H2O on the right hand side.

3Ca(OH)2 + 2H3PO4 ---> Ca3(PO4)2 + 6H2O
It's now a balanced equation. Note how we treated the PO4 ion as a single species to be balanced.

Example 4

FeCl3 + NH4OH ---> Fe(OH)3 + NH4Cl

Answer » The most obvious error is that there are three chlorines on the left but only one on the right, and the chlorine is in a single chemical species on each side . Put a 3 in front of the NH4Cl on the right hand side.

FeCl3 + NH4OH ---> Fe(OH)3 + 3NH4Cl

The next most obvious unbalanced part is that there are now three NH4 groups on the right but only one on the left hand side. So put a 3 in front of the NH4 on the left.

FeCl3 + 3NH4OH ---> Fe(OH)3 + 3NH4Cl

And if you count up the atoms on each side, you will see that this is now a balanced chemical equation.


Example 5

S8 + F2 ---> SF6

Answer » If we start by balancing the fluorine, we know that as soon as we try to balance the sulfur, we will have to alter the fluorine again. So start with the sulfur. There are eight of them on the left but only one on the right. Put an 8 in front of the SF6

S8 + F2 ---> 8SF6

And now we can see that there are 48 fluorines on the right and only two on the left, so put a 24 in front of the F2 on the left.

S8 + 24F2 ---> 8SF6

And that chemical equation is now balanced. Check this by counting the number of atoms of each type on each side.

Example 6

C2H6 + O2 ---> CO2 + H2O

Answer » There are two carbons on the left but only one on the right, and the carbon is in a single chemical species on each side. Put a 2 in front of the CO2 on the right hand side.

C2H6 + O2 ---> 2CO2 + H2O

Because the oxygen is in two compounds on the right, we will look at the hydrogen next as it is in one compound on each side of the equation. There are six hydrogens on the left and two on the right - put a 3 in front of the H2O on the right

C2H6 + O2 ---> 2CO2 + 3H2O

Now we have two oxygens on the left and seven on the right. Put a 3 1/2 in front of the O2 on the left.

C2H6 + 3 1/2O2 ---> 2CO2 + 3H2O

BUT we don't like having a half in a chemical equation, so multiply every coefficient on both sides by two.

2C2H6 + 7O2 ---> 4CO2 + 6H2O

And this is now a balanced chemical equation.


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