The law of conservation of mass states that the amount of matter of each element before the reaction is equal to the amount of matter of each element after the reaction. Thus, the left and right sides of the chemical equation must have the same number of atoms of one or another element. The chemical equation must be electrically neutral, that is, the sum of the charges on the left and right sides of the equation must add up to zero. One way to equalize the number of atoms in a chemical equation is to select coefficients by trial and error.
We write down the equation of reactions.
P2O5 + 3H2O = 2H3PO4
We calculate the sum of all the coefficients: 1 + 3 + 2 = 6.
The sum of the odds is 6.
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