I don't excel at chemistry at all, but this one is pretty simple, really. Wikipedia is your friend.
Simply put: the empirical formula is the ratio of the numbers of atoms (or moles) of each element in a compound. The molecular formula incorporates the actual number of atoms of each element in the compound.
A good example is glucose. The molecular formula C6H12O6, which is the actual number of each type of atom in the molecule. The empirical formula can be written as CH2O, since there are equal numbers of C and O atoms, and twice as many H atoms (think of it as being similar to simplifying a fraction using least common denominators).
Another example is hydrogen peroxide. The molecular formula is H2O2, but the empirical formula is HO.
GIVEN:
184g of a compound
69.6% S by mass
30.4% N by mass
FIND:
empirical formula
molecular formula
SOLUTION:
Atomic weight of S = 32.065 ~ 32
Atomic weight of N = 14.0067 ~ 14
For S:
184g * 69.6% = 128.064g S / 32 g/mol = 4.002 mol S
For N:
184g * 30.4% = 55.936g N / 14 g/mol = 3.995 mol N
Since the number of S atoms is approximately equal to the number of N atoms, the
empirical formula is SN.
The most likely
molecular formula for this compound is
Tetrasulfur Tetranitride, or S4N4 - but it could just as easily be Disulfur Dinitride (S2N2), or any other molecule with an equal number of S and N atoms.