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

Question

jfraser · Answer

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

anonymous · Answer

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

anonymous · Answer

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).