% Composition and Empirical/Molecular Forms

We discussed percent composition in class last week. These problems are solved by simply taking the mass of each element in the compound and dividing it by the mass of the entire compound and then multiplying it by 100. You can then use the percent composition of that element and calculate the mass of it in a certain size sample by multiplying the % comp by the mass of the sample. An example of this can be found in the following video. https://www.youtube.com/watch?v=lywmGCfIUIA
We also talked about empirical form vs. molecular form. Empirical formula is the lowest number ratio, but molecular gives the actual number of atoms in the compound. Below is a chart to help show the differences.

To find the empirical formula we used this process: 

Examples of this can be found on the following link. http://www.chemteam.info/Mole/EmpiricalFormula.html

Using empirical formula you can find the molecular formula by taking gram formula mass and dividing it by the empirical formula mass to get a whole number, and then distributing it to the empirical formula. Some practice problems can be found on the following links. http://www.chemteam.info/Mole/Empirical-MolecFormulas.html and http://chemistry.about.com/od/chemistry-test-questions/tp/Molecular-Formula-Practice-Test-Questions.htm