why do you assume 100 g of a substance when given percentages?

Question

when given a problem and the elements of a compound are given in percentages, i don't understand why you can just assume that it's 100g of the compound. why couldn't it be 200g or 700g of the substance, which would all give you a different answer.

Answer 1

It wouldn't give you a different answer if you did it correctly. We always assume 100 g because the definition of PERCENT is parts per 100, not parts per 200 or parts per 700. It just makes our calculations much easier.

If we had, let's say, 23% C (carbon).

If we assume 100 grams of substance, we would have 23 g of C, or 0.23 g of C per g of substance

If we assume 200 grams of substance, we would have 46 g of C, or 0.23 g of C per g of substance.

No difference on a per gram of substance basis.