# Question Video: Determining the Number of Additional Oxygen Molecules Needed to Balance an Equation Chemistry • 7th Grade

Consider the following equation: CH₃CH₂OH + O₂ ⟶ 2 CO₂ + 3 H₂O. How many more oxygen molecules are required to balance the chemical equation?

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### Video Transcript

Consider the following equation: CH3CH2OH plus O2 react to produce two CO2 plus three H2O. How many more oxygen molecules are required to balance the chemical equation?

A chemical equation is balanced when the number of atoms of each element is the same on both sides of the reaction. Chemical equations can be balanced by adding coefficients. Coefficients are numerical values written before a species in a chemical equation. In the equation provided, the two written in front of CO2 and the three written in front of H2O are coefficients. The question asks how many more oxygen molecules are required to balance this chemical equation. So we need to determine what coefficient should be placed in front of O2 in the chemical equation in order for the equation to be balanced.

To determine this coefficient, let’s take a closer look at the reaction equation. We can see that all of the reactants and products are composed of either carbon, hydrogen, or oxygen atoms. So let’s start by making a list of the number of carbon, hydrogen, and oxygen atoms on both sides of the reaction equation. The first reactant contains two carbon atoms, a total of six hydrogen atoms, and one oxygen atom. Also on the reactant side is a molecule of oxygen, which contains two oxygen atoms. So, on the reactant side, there’s a total of two carbon atoms, six hydrogen atoms, and three oxygen atoms.

Now let’s look at the first product, carbon dioxide. A molecule of carbon dioxide contains one carbon atom and two oxygen atoms. But in the chemical equation, a coefficient of two is placed in front of carbon dioxide. This means that the reaction produces two molecules of carbon dioxide. So the total number of carbon atoms from the two molecules of carbon dioxide is two, and the total number of oxygen atoms is four. The reaction also produces three molecules of water. One molecule of water contains two hydrogen atoms and one oxygen atom. So three molecules of water will have a total of six hydrogen atoms and three oxygen atoms. So, on the product side, there are a total of two carbon atoms, six hydrogen atoms, and seven oxygen atoms.

The number of carbon atoms and hydrogen atoms are the same on both sides of the reaction equation. So these atoms are balanced. But the number of oxygen atoms are not the same. So the oxygen atoms are unbalanced. To balance the oxygen atoms, we need to add more oxygen molecules. Two oxygen molecules would contain four oxygen atoms. This would bring the total number of oxygen atoms on the reactant side to five. But the oxygen atoms would still be unbalanced. Three oxygen molecules would contain six oxygen atoms. This would bring the total number of oxygen atoms on the reactant side to seven, and the reaction equation would be balanced.

So now we know that three oxygen molecules are required to balance the chemical equation. But the question asks how many more oxygen molecules are required. The provided equation only contained one oxygen molecule. Two additional molecules of oxygen were added in order to balance the chemical equation. So, in order to balance the provided chemical equation, two more oxygen molecules are required.