How many grams of Cl are in 38.0g of CF2Cl2? I really want to figure out how to do it. I can't seem to understand any of this Chem homework and it's getting me frustrated, please help!

Answer 1

You work through these things logically. The best advice I have for beginning students is to think "moles".
How many mols CF3Cl2 are in 38.0 g. I will estimate all of these numbers; you should do them more accurately.
mols = grams/molar mass = 38/121 = approx 0.3
mols Cl in CF2Cl2 is twice that because there are 2 Cl atoms in 1 molecule of CF2Cl2. So mols Cl atoms is about 0.6
Then convert mols to g. g = mols x atomic mass = 0.6 x 35.5 = about 22 g Cl.

There is a shortcut you can use when talking about the mass of something in the SAME molecule. That is
38g x (mass of 2Cl/mass CF2Cl2) = 38 x (2*35.5/121) = about 22.
Basically this last method is just a percent problem.
%Cl in CF2Cl2 = (2*mass Cl/mass CF2Cl2)*100 = (235.5/121)*100 = about 58.7%
Then 38g x 0.587 = about 22 g.