Empirical + Molecular Formula 1/12

Empirical Formula = gives the lowest term ratio of atoms or molecule in the formula!

All ionic compounds are empirical formulas!C4H10 (Butune) > molecular formula
C2H5                 > empirical formula

Ex.) Consider that we have 10.87g of Fe and 4.66g of O. What is the empirical foumula?
#1 Convert g > mole
Fe: 10.87g/ (55.8) > mole = 0.195g/mol
O : 4.66/ (16) > mole = 0.291g/mol#2 Divide both by the smallest molar amount.
Fe: 0.195 / 0.195 = 1
O : 0.291 / 0.195 = 1.49 > 1.5
(as 1.5 can't be in the empirical formula, we have to multiply it into whole number)  O : 1.5 x 2 > 3 , and the other one also have to multiply the same amount, which is "2" in this situation.
Fe : 1 x 2    > 2#3 Scale ratios to whole numbers
Fe2O3
DONE! : )


Second part:


Molecular Formula:it's a multiple of the empirical formula and shows the actual number of atoms that comine to form a molecule to calculate multiple:

Ex.) A molecule has an empirical formula of C2H5 and a molar mass of 85g/mol. What is the molecula formular?

#1 : molar mass of C2H5 = 12 x 2 + 5 = 29g/mol

N=  (58g/MOL) / (29) = "2"
#2:   "2" x C2H5 = C4H10 
DONE!

Let's take a look at this video ! As Ms.Chen said, there won't have any question hardar than this one, once you can handle this one.  You will be fine I bet : )