The Empirical & Molecular Formula
The Empirical Formula
The empirical formula is the simplest formula, or the simplest whole-number ratio, of the atoms of the elements found in a compound. If you were to take the subscripts of the elements in a compound and simplify them to their simplest form, you would get the empirical formula of that compound. However, keep in mind that many different compounds can have the same empirical formula.
Examples
[Glucose] C6H12O6 ---> CH2O
[Putrescine] C4H12N2 ---> C2H6N
[Dinitrogen Tetroxide] N2O4 ---> NO2
[Dinitrogen Tetrahydride] N2H4 ---> NH2
Examples
[Glucose] C6H12O6 ---> CH2O
[Putrescine] C4H12N2 ---> C2H6N
[Dinitrogen Tetroxide] N2O4 ---> NO2
[Dinitrogen Tetrahydride] N2H4 ---> NH2
Calculating the Empirical Formula
In order to calculate the empirical formula, do the following steps.
- Find the Percent Composition
- Convert the Percent Composition to Grams (in this case, simply change % to g since it's assumed to be out of a 100g sample)
- Calculate the number of moles of each element by converting each element from grams to moles
- Divide every molar quantity you calculated by the smallest molar quantity that you calculated
- After dividing, the numbers that you calculated represent the ratio of atoms (the subscripts) found in your empirical formula
- Write out the empirical formula
The Molecular Formula
The molecular formula of a compound is the actual formula for the compound. This formula shows you exactly how many atoms of each element are present in the compound, unlike the empirical formula, which is in the simplest form. However, it is possible that the molecular formula of a compound is the empirical formula of that compound.
Examples
[Formaldehyde] CH2O (The molecular formula of formaldehyde is also the empirical formula of formaldehyde, as it's the simplest form.)
[Chloroform] CHCl3 (The molecular formula of formaldehyde is also the empirical formula of formaldehyde, as it's the simplest form.)
[Water] H2O (The molecular formula of formaldehyde is also the empirical formula of formaldehyde, as it's the simplest form.)
[Glucose] C6H2O6
[Putrescine] C4H12N2
[Dinitrogen Tetroxide] N2O4
Examples
[Formaldehyde] CH2O (The molecular formula of formaldehyde is also the empirical formula of formaldehyde, as it's the simplest form.)
[Chloroform] CHCl3 (The molecular formula of formaldehyde is also the empirical formula of formaldehyde, as it's the simplest form.)
[Water] H2O (The molecular formula of formaldehyde is also the empirical formula of formaldehyde, as it's the simplest form.)
[Glucose] C6H2O6
[Putrescine] C4H12N2
[Dinitrogen Tetroxide] N2O4
Calculating the Molecular Formula
In order to calculate the molecular formula, do the following steps.
- Find the Empirical Formula
- Find the molar mass of the empirical formula
- Find the molar mass of the molecular formula (should be given)
- Divide the molar mass of the molecular formula by the molar mass of the empirical formula
- Multiply your answer in step 4 by each subscript in the empirical formula to determine the subscript of each element in the molecular formula