• 1. Percent to mass

If percent composition is given, change each percentage to the mass of each element in grams.
If mass is given, skip the step 1.


• 2. Mass to mole

Convert the amount of each element in grams to its amount in moles.


• 3. Divide by smallest number of moles.

Divide each of the found values in step 2 by the smallest number of moles.


• 4. Multiply by whole numbers until result is in whole numbers.

If necessary, multiply these numbers by integers in order to get whole numbers. If an multiplication is done to one of the numbers, it must be done to all of the numbers.
If step 3 is resulted in whole numbers, skip step 4.

Example:

A 200 gram chemical compound contains 71.5% of calcium and 28.5% of oxygen by mass. Calculate the emperical formula for this compound.

• 1. Percent to mass

A 100 gram chemical compound contains 71.5 grams of calcium and 28.5 grams of oxygen.
A 200 gram chemical compound contains 143 grams of calcium and 57 grams of oxygen.


• 2. Mass to mole

Find the masses of each element. Lokk at periodic table or periodic chart to find the masses of each element.
Mass of calcium = 40.1 grams.
Mass of oxygen = 16.0 grams.
Find mole amounts by dividing the masses obtianed in step1 by elemental masses.
$\mathrm{calcium}=\frac{\mathrm{143\; grams}}{\mathrm{40.1\; grams/part}}=\mathrm{3.56\; parts}$
$\mathrm{oxygen}=\frac{\mathrm{57\; grams}}{\mathrm{16.0\; grams/part}}=\mathrm{3.56\; parts}$


• 3. Divide by smallest number of moles.

calcium = 3.56
oxygen = 3.56
Ratio – 1 Ca: 1 O
Empirical Formula = CaO


• 4. Multiply by whole numbers until result is in whole numbers.

As step 3 is resulted in whole numbers, skip step 4.
Empirical Formula = CaO

• 1. Find the empirical formula
• 2. Calculate the Empirical Formula Mass
• 3. Divide the molar mass by the Empirical Formula Mass
• 4. Multiply empirical formula by factor obtained in step 3

Example:

The empirical formula of a chemical compound is HO and the molar mass is 32.02 grams. What is the molecular formula for this compound?

• 1. Find the empirical formula

It is given that the empirical formula is HO.
It is given that the molar mass is 32.02 grams.


• 2. Calculate the Empirical Formula Mass

Find the masses of each element. Look at periodic table or periodic chart to find the masses of each element.
Mass of hydrogen = 1.01 grams.
Mass of oxygen = 16.0 grams.
The empirical mass is 1.01g + 16.00g = 16.01 grams.


• 3. Divide the molar mass by the Empirical Formula Mass

$\mathrm{factor}=\frac{\mathrm{32.02\; grams}}{\mathrm{16.01\; grams/part}}=2$


• 4. Multiply empirical formula by factor obtained in step 3

Molecular Formula = (Empirical Formula)_n
Molecular Formula = (HO)₂
Empirical Formula = H₂O₂