Tetrachloroethane is a valuable nonflammable solvent. it's percent composition is 14.31% carbon, 1.20% hydrogen, and 84.49% chlorine. what is the empirical formula of this compound?

First take all percents and make them grams. Since you're not given a overall molar mass you can assume it is 100 and therefore the percents are their masses.
So you have 14.31g Carbon, 1.2g Hydrogen, and 84.49g of Chlorine. Next you divide each by their molar masses to get moles of each.

Carbon= 14.31g       Hydrogen= 1.2g         Chlorine= 85.49g
               12.01g                         1.01g                         35.45g
          = 1.19moles                = 1.188moles          = 2.411moles

Next you divide each of those numbers by the smallest, in this case, Hydrogen.
Thus, 
    Carbon= 1.19moles     Hydrogen= 1.188moles   Chlorine= 2.411moles
                   1.188moles                      1.188moles                   1.188moles 
             =1.002                               =1                                =2.02
These are all close enough to round, so your final empirical formula is: CHCl2
Hope that helps!!
 

And a) oxygen i believe

Where is the following?