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Because there are 3 carbons on the left and only 1 on the right of the equation, we can balance this by adding a coefficient to the single carbon on the right. Thus, it will become, C3H8 + O2 --> H2O + 3CO2 The coefficient 3 in front of carbon on the right side indicates 3 carbon atoms just as the subscript 3 on the left side indicates 3 carbon atoms. In a chemical equation, you can change coefficients, but you should never alter the subscripts.
Balance the hydrogen atoms next. You have 8 on the left side. So you'll need 8 on the right side. C3H8 + O2 --> 4H2O + 3CO2 On the right side, you now added a 4 as the coefficient because the subscript showed that you already had 2 hydrogen atoms. When you multiply the coefficient 4 times the subscript 2, you end up with 8.
Let's finish by balancing the oxygen atoms. Because you've added coefficients to the molecules on the right side of the equation, the number of oxygen atoms has changed. You now have 4 oxygen atoms in the water molecule and 6 oxygen atoms in the carbon dioxide molecule. That makes a total of 10 oxygen atoms.
Add a coefficient of 5 to the oxygen molecule on the left side of the equation. You now have 10 oxygen molecules on each side. C3H8 + 5O2 --> 4H2O + 3CO2. The carbon, hydrogen, and oxygen atoms are balanced. Your equation is complete.
Is it all as easy as I made it look? Well, yes and no. Yes, it should work all the time. No, sometimes you need to do some tricks to find the right numbers to add into the equation. For example, what happens when you do the inventory, and you find that there are two atoms of element X on the left side of the equation and three on the right. How can you make those match?
When you run into this problem, find the lowest common denominator of those two numbers, and then put the numbers in front of those two boxes which allow the inventory on both sides to match. In the element X example, the lowest common denominator of two and three is six, so you'd put a "3" in front of the molecule on the left, and a "2" in front of the one on the right. Element X will then match up, and you can use a new inventory to see what else needs to be done.
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