What is Empirical formula:
- Empirical formula gives the LOWEST TERM RATIOS of atoms or moles
- All formula of ionic compounds are empirical
- e.g. NaCl, MgO, Al2O3
How to determine the Empirical formula given the mass of each atoms?
2 simple steps:
- Convert all the mass of elements (in grams) to moles
- Find the simplest whole number ratio between these amounts
e.g. Determine the empirical formula of a 100 g compound of phosphorus and oxygen that contains a 43.64% of phosphorus by mass.
Solution:
In 100 g, there are 43.64 g of phosphorus and 56.36 g (100 - 43.64 g) of oxygen.
Amount of phosphorus = 43.64 g / 30.97 g mol-1 = 1.409 mol
Amount of oxygen = 56.36 g / 16.00 g mol-1 = 3.523 mol
The whole number ratio of phosphorus to oxygen bay be found by dividing through by the smaller number:
Ratio of P : O = 1.409 : 3.523 = 1 : 2.5 (dividing by 1.409) = 2 : 5 (multiplying by 2)
Therefore, the empirical formula is: P2O5
What is Molecular formula:
- Molecular formula gives all atoms which makes up a molecule
- Ionic or covalent compound formulas can be molecular
- e.g. C3H8, Na2CO3
IMPORTANT EQUATIONS!!!
Molecular formula (MF) = Empirical formula (EF) x Whole Number (N)
MF mass = EF mass x N
Mass of one mole = EF mass (in grams) x N
Tool kit
Here is an awesome online empirical formula calculator. You can check if your answers are correct by typing in the mass percentage of element and the name of the element.
Practice questions
If you want to practice on solving empirical formula problems, check out these:
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