We have to balance this equation:
__ C₃H₈O + __ O₂ ----> __ CO₂ + __ H₂O
The first step to balance the equation is to count the number of atoms of each element that we have on each side of the equation:
__ C₃H₈O + __ O₂ ----> __ CO₂ + __ H₂O
H: 8 H: 2
C: 3 C: 1
O: 3 O: 3
Let's start balancing the H. We have 8 atoms of H on the left and 2 atoms of H on the right. We can change the coefficient in front of H₂O and write a 4 there to get 8 atoms of H on the right side of the equation.
__ C₃H₈O + __ O₂ ----> __ CO₂ + 4 H₂O
H: 8 H: 8
C: 3 C: 1
O: 3 O: 6
We balanced the H. Now we can balance the C. We have 3 atoms of C on the left and just 1 atom on the right. The coefficient for CO₂ must be a 3.
__ C₃H₈O + __ O₂ ----> 3 CO₂ + 4 H₂O
H: 8 H: 8
C: 3 C: 3
O: 3 O: 10
Finally we can balance the O. We have 3 atoms of O on the left and 10 atoms of O on the right. We have two options, we can change the coefficient for C₃H₈O or O₂. It is easier to work with the coefficient in front of O₂ because if we change the other one we will change the number of C and H atoms.
The coefficient for O₂ must be 9/2.
__ C₃H₈O + 9/2 O₂ ----> 3 CO₂ + 4 H₂O
H: 8 H: 8
C: 3 C: 3
O: 10 O: 10
Now the equation is balanced but we can't have fractions. The coefficients are entire numbers. How can we convert 9/2 into an entire number? We can multiply all the coefficients by 2.
2 C₃H₈O + 9 O₂ ----> 6 CO₂ + 8 H₂O
H: 16 H: 16
C: 6 C: 6
O: 20 O: 20
So the answer to our problem is:
Answer: 2 C₃H₈O + 9 O₂ ----> 6 CO₂ + 8 H₂O