Empirical formula

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 The empirical formula of a substance is its simplest whole number ratio. The Molecular Formula is a multiple of the Empirical Formula.

To calculate Empirical Formula:

 Mass (g) / Relative Atomic Mass (Ar)  → This will give you the number of moles.

 You then divide the number of moles by the smallest number reulting from your previous calculations, and that will give you the ratio for the empirical formula.


Example:

You have 9.62 g of Carbon, 1.60 g of Hydrogen and 4.28 g of Oxygen. What is the empirical formula of this CHO substance?

C → 9.62 / 12.01 (Ar) = 0.80 (number of moles)

H → 1.60 / 1.01 = 1.60

O → 4.28 / 16.00 = 0.27

Smallest value = 0.27, so:

C → 0.80 / 0.27 = 2.96 

H → 1.60 / 0.27 = 5.93

O → 0.27 / 0.27 = 1


Empirical formula uses whole numbers, so you must round the numbers obtained to the closest whole number. This gives us an empirical formula of:

C3H6O

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