Question

The percentage by mass of elements in a particular pure substance will vary from sample to sample.

Answer 1

Do you think it matters whether you buy name-brand sugar or generic sugar?

Answer 2

soo........what's the question???

Answer 3

There are two facts you need here: the first is the definition of the word "pure" in "pure substance." The second is that the properties of a substance are unique determined by its chemical formula. For example, table salt is NaCl. Nothing else behaves exactly like table salt, and anything with even a slightly different chemical formula doesn't behave exactly like table salt (much to the sorrow of those trying to cut down on their salt intake).