Chemical Composition Pre-Test

Today we took our unit pretest on chemical composition. The test contained questions regarding density and molecules. I was uber confused on the questions asking how many molecules in a specific mass or volume. Some questions I tried and re-tried calculating, but it didn't work, so I just guessed. This unit seems to be one of the harder units. I hope those confusing concepts will be made clear after the lectures. One concept I had not ever heard of were empirical formulas.

http://jencrazyscience.blogspot.com/2012/09/chemical-composition-of-earth.html

I am very curious as to what and how to calculate problems according to molecular mass. I also need to review the poly-atomic ions. Additionally, I am also very interested as to what the project/lab will be for this unit.