Determining Empirical/Molecular Formulas

What Is An Empirical/Molecular Formula?

An empirical formula is a chemical formula showing the ratio of elements in a compound rather than the total number of atoms. It is the simplest whole number non-reducible ratio formula for a molecular formula or compound.

A molecular formula is a formula indicating the supposed molecular constitution of a compound, commonly consisting of a series of letters and numbers comprising the atomic symbols of each element present in a compound followed by the number of atoms of that element present in one molecule of the substance.

Determining Empirical/Molecular Formulas

How to find the Empirical Formula from Element Proportions

We can determine the empirical formula by using the proportion of each element in the compound data. A compound is composed of 40% carbon, 6.67% hydrogen, and 53.3% oxygen. Find the empirical formula for this compound knowing that H = 1 g/mole, O = 16 g/mole and C = 12 g/mole.

Assume 100 g of the substance.

Then convert the grams of each element to moles by dividing by their molar mass:
40/12 = 3.33 moles C
6.67/1 = 6.67 moles H
53.3/16 = 3.33 moles O

Divide each of the three mole figures by the lowest of the three in order to simplify the mole ratio.
3.33/3.33 = 1 moles C
6.67/3.33 = 2 moles H
3.33/3.33 = 1 moles O

Well, that is 1 C, 2 H and 1 O.
The resulting SIMPLEST WHOLE NUMBER empirical formula: CH2O

NOTE: In this case all mole figures represent whole numbers, so we can say that this is the SIMPLEST WHOLE NUMBER formula.
If one of the numbers was (say) 1.5, we have to multiply all the mole figures by 2.

How to find the Empirical Formula From Element Mass

We can determine the empirical formula by using mass of each element in the compound data. A compound is composed of 5.045g of carbon, 0.847g of hydrogen, and 3.36g of oxygen. Find the empirical formula for this compound knowing that H = 1 g/mole, O = 16 g/mole and C = 12 g/mole.

First, convert the grams of each element to moles by dividing by their molar mass:
5.045/12 = 0.42 moles C
0.847/1 = 0.847 moles H
3.36/16 = 0.21 moles O

Divide each of the three mole figures by the lowest of the three in order to simplify the mole ratio.
0.21/0.21 = 1 moles O
0.42/0.21 = 2 moles C
0.847/0.21 = 4 moles H

Well, that is 1 O, 2 C and 4 H.
The resulting SIMPLEST WHOLE NUMBER empirical formula:  C2H4O

NOTE: In this case all mole figures represent whole numbers, so we can say that this is the SIMPLEST WHOLE NUMBER formula.
If one of the numbers was (say) 1.5, we have to multiply all the mole figures by 2.

How to find Molecular Formula from the Empirical Formula

Once you have the empirical formula, you an find the molecular formula if the molar mass of the substance is known. Find the molecular formula of a compound whose empirical formula is CH2O and molar mass is 60 g/mole.

First, calculate the molar mass of the empirical formula:
1(12 g/mole C) + 2(1 g/mole H) + 1(16 g/mole O) = 30 g/mole CH2O

Next, Divide the molar mass of the substance by the molar mass of the empirical formula:
60/30 = 2

Finally, multiply the number of atoms of each element in the empirical formula by this ratio.
1(2) = 2 C
2(2) = 4 H
1(2) = 2 O

The resulting molecular formula is C2H4O2.

Using a Tool

You can also use our empirical formula calculator.