Empirical and Molecular Fomula
It gives the lowest term ratio of atoms (or moles) in a formula.
Ex. C4H10 (butane) --> molecular fomula
Reduce the subscripts to lowest terms to get the empirical formula
C2H5 --> empirical formula
Ex. Consider that we have 10.87 grams of Fe and 4.66 g of O. What is the empirical formula?
First, we must convert the grams to moles.
10.87g x 1mole = 0.195 moles of Fe
4.66g x 1 mole = 0.291 moles of O
Now, we divide the mole amount by the smallest mole which is 0.195
Fe: 0.195 = 1
O: 0.291 = 1.49 = 1.5
Next, we must get those two numbers to the smallest whole number
Fe 1 x 2 = 2
O 1.5 x 2 = 3
The empirical formula is: Fe2O3
Ex. A compound contains 31.9% of K, 28.9% of Cl, and 39.2% of O. What is the empirical formula?
*assume we have 100g*
31.9 x 1 mol = 0.816 moles of K
28.9 x 1 mol = 0.814 moles of Cl
39.2 x 1 mol = 2.45 moles of O
lowest mole: 0.814
0.816 = 1.00
0.814 = 1
2.45 = 3
Empirical formula: KClO3
What is the molecular formula?
It is a multiple of the empirical formula and shows the actual numbers of atoms that combine to form a molecule.
To calculate the multiple, we use this formula:
n = molar mass of the compound
molar mass of the empirical formula
Ex. A molecule has an empirical formula of C2H5 and the molar mass is 58 g/mol. What is the molecular formula?
Molar mass is C2H5 = 58 g/mol
Molar mass of the empirical formula (total molar mass of all elements in the compound):
C2 = 12 x 2 = 24
H5 = 1 x 5 = 5
C2H5 = 29 g/mol
58 g/mol = 2
2 x C2H5 = C4H10
Video about the empirical formula:
Video about the molecular formula:
Posted by TheChemGuys