Empirical and Molecular Formulas
Empirical Formula: smallest whole # ratio of atoms in a compound
Molecular Formula: total number of atoms in a compound
Molecular (total)
C6H12O6
N2O2
C4H12Cl2
CH2O
NO
C2H6Cl
C3H12
C4H10O
Si4H12
Molecular Mass
(molar mass of molecule)
Empirical (simple)
Empirical Mass
(molar mass of simple)
Practice: Complete the following table!
Molecular (total)
Empirical (simple)
How to Calculate Empirical Formulas: Here’s a helpful rhyme to assist you!
% to mass
Mass to mole
Divide by small
Multiply til whole!
Example: A compound has 11.2% H and 88.9% O. What is its empirical formula?
1. % to mass (Pretend it’s out of 100 grams)
11.2 % hydrogen = _____________
88.9 % oxygen = _______________
2. Mass to mole (Use the mole map: divide by molar mass)
11.2 grams hydrogen / _____________ = _____________
88.9 grams oxygen / _____________ = _____________
3. Divide by small (find smallest answer in step 2, divide all answers by that #)
**5.56 is the smallest answer in step 2***
11.1 mol hydrogen / _____________ = _____________  rounds to ______
5.56 mol oxygen / _____________ = _____________
*If the number is a decimal ending in .1 or .2  round down to a whole number
**If the number is a decimal ending in .8 or .9  round up
4. Multiple til whole! Every number needs to be a whole number
H  2 and O  1
Already whole! We don’t need this step.
*If the number is a decimal ending in .5  multiply ALL numbers by 2
**If the number is a decimal ending in .33 or .66  multiply ALL numbers by 3
5. LAST STEP: Use whole numbers as subscripts in formula. H  2 and O  1 so the formula = H2O
Empirical Formula Practice Problems
1. One compound contains 5.717 g oxygen and a phosphorus content of 4.433g. What is the empirical formula of this
compound?
2. A given compound is shown to contain 32.28% sodium, 22.65% sulfur, and 44.99% oxygen. Find the empirical
formula of this compound.
3. What is the empirical formula of a compound containing 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen?
4. Given 13.5 g of calcium, 10.8 g oxygen, and 0.675 g of hydrogen, what is the empirical formula of this compound?
How to get the Molecular/True Formula
Ex: What is the molecular formula for a compound with the empirical formula H2O and a molecular mass of 54 g?
1. Step 1: GET 3 THINGS (empirical formula, empirical mass, and molecular mass)
2. STEP 2: Molecular Mass
Empirical Mass
=
3. STEP 3: Take answer x Empirical Formula = Molecular Formula!
Molecular Formula Practice Problems
1. The empirical formula of a compound of phosphorus and oxygen was found to be P2O5. Experimentation shows that
the molar mass of this compound is 283.89g/mol. What is the compound’s molecular formula?
2. A sample of a compound with a formula mass of 34.00amu is found to consist of 0.44g H and 6.92g O. Find its
molecular formula.