Question

What is the mass percent of `"Cl"` in `"CCl"_4`?

Answer 1

Mass percent is a quantity that tells you the amount of something in something else in a relative manner.

The nice thing about mass percent is that it is an intensive property (does not depend on the absolute mass present). In other words, the mass percent does NOT change if you have, let's say, `"100 g"` of sample vs. `"2 g"` of sample.

That means you can figure out the mass percent simply by knowing how many chlorines there are relative to the molecule itself, and what the molar masses of each component are!

What you can do is the following:

1. Recall or look up the molar masses of `"Cl"` (35.453) and `"C"` (12.011) in `"g/mol"`.
2. Use this simple formula:

   `"Atomic Mass of Atom of Interest"/"Molecular Mass" xx 100%`
   (In this case, you can actually ignore the mass units if you wish, because you'll end up canceling them out anyways.)

Using this information, we just have:

`(35.453 xx "4 equiv. Cl")/(12.011xx"1 equiv. C" + 35.453xx"4 equiv. Cl") xx 100%`

`= 141.812/153.823 xx 100%`

`~~ color(blue)(92.19% "Cl")`