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To balance any chemical equation we adjust the coefficients until the same number of atoms appear on the reactant and product side of the equation. This is not always an easy task! Consider the following reaction in which nitric oxide (NO) reacts with hydrogen (H2) to produce ammonia (NH3) and water. This reaction is used to synthesize ammonia for fertilizer and other applications. 
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You cannot change subscripts in chemical formulas because this would change the identities of the substances in the reaction and make it a different reaction. For example, it's impossible to change H2 to H5 to get three more hydrogen atoms and balance the equation because H5 does not exist. Even if H5 did exist it would be a different compound than H2.
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You can only change the coefficients in whole-numbers to adjust the number of each molecule in the reaction. For example we can change H2 into 3H2 which increases the number of hydrogen atoms from two to six. We cannot use a fractional coefficient such as 2.5 - that would divide a molecule and create a different compound.
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The balanced equation tells us that when we combine 2 moles of nitric oxide with five moles of diatomic hydrogen we can obtain 2 moles of ammonia and 2 moles of water. This is the recipe for making ammonia from nitric oxide and hydrogen. It correctly represents the amount of each substance and satisfies mass conservation because the total number of each type of atom is equal on both sides of the equation.
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