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We left oxygen to be balanced last since it is involved in the most number of compounds. Also, oxygen is a pure substance so changing the coefficient specifically on O2 will not affect the other elements that are already balanced. Let’s count the oxygen atoms as they appear in the equation below. 
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By adding a coefficient of 6 to O2 on the reactant side, the reaction is balanced. The 18 moles of oxygen on the reactant side balance the 18 moles of oxygen on the product side. You have now addressed all atoms in the reaction: H, C and O. Before you decide that you are done, check to see that the equation is balanced. Count the number of atoms for each element on either side of the equation. This equation is correctly balanced because the number of atoms for each element is the same on both sides of the equation.
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The final step is to make sure the coefficients are reduced to the smallest whole numbers possible. The same balanced reaction could be written with 2 moles of glucose and 12 moles of everything else. This would not be the smallest whole numbers because we can divide them all by two to get a simpler form with smaller coefficients that is also balanced.
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Here is a summary of how to use a structured method to balance chemical equations.
- Write the unbalanced chemical equation.
- Find the element that occurs in the fewest number of compounds on both sides of the equation and balance it first. If there is a tie between more than one element, balance them in any order.
- Move on to the rest of the elements. Balance pure elements last.
- Re-count the number of each element on both sides to make sure they are equal.
- Make sure the coefficients cannot reduce. Coefficients must be the smallest possible whole numbers.
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