Empirical & Molecular Formula

Empirical Formula (EF)

EF: the simplest whole number ratio of each type of atom in a compound

How to calculate EF from the masses of elements in a compound:

1) find moles of each element using mass/Ar

2) divide all the mole values by the smallest number of moles

3) write the simplest whole-number ratio

*Example on flashcard

Calculating EF from % composition:

→ Assume there are 100g of the compound, so each percentage becomes a mass in grams (g)

1) find moles of each element using mass/Ar

2) divide all the mole values by the smallest number of moles

3) write the simplest whole-number ratio

Molecular Formula (MF)

MF: the actual number of atoms of each element in a compound

• We can obtain the MF for any compound from its EF if we know either the molecular weight or molar mass of the compound

  • the subscripts in the MF of a substance are always a whole # multiples of the subscripts in its EF

Steps to find MF from EF and relative formula mass (Mr)

1) work out the EF mass

2) use: whole # multiple = Mr/EF mass

3) multiply all the atoms in the EF by this number