Which of the following must be the same before and after a chemical reaction?
Answer: The sum of the masses of all substances involved and the number of atoms of each type involved. WHY?
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The law of conservation (of mass) (or matter) says mass (or matter) can't be created or destroyed. Therefore, if you start with a total of x grams you must end up with a total of x grams. And if you start with 14 N atoms or 26 O atoms you must end up with 14 N atoms and 26 O atoms. They may be arranged differently from start to end but the number must be the same.
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