1. you perform a combustion analysis on a 255 mg sample of a substance that contains only c, h, and o, and you find that 561 mg of co2 is produced , along with 306 mg of h2o.

a. if the substance contains only c, h, and o, what is the empirical formula?

b. if the molar mass of the compound is 180g/mol, what is the molecular formula of the compound?

We let x, y, and z be the number of C, H, and O atoms in the formula. Through inventory, it may be noted that the amount of the total product is equal to 867 mg while the given reactant is only 255 mg, the amount of O2 is equal to 612 mg equal to 38.25 mmol O. 
By elemental balance. 
     (C)             255(x/255) = (561)(12/44)   ; x = 153
     (H)              255(y/255) = (306)(2/18)    ; y = 34
     (O)              255(z/255) + (612) = (561)(32/44) + (306)(16/18)
                                           z = 68
The empirical formula is C4.5HO2. 

(b) Given that the molar mass is 180, the molecular formula is C9H2O4. 

Molarity is moles of solute over liters of solution. idk if that is what you are talking about, but you can convert moles of solute to grams of solute using dimensional analyzes by using molar mass found on the periodic table.