Empirical formula

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

For example:

The molecular formula of butanoic acid is C4H8O2 whilst the empirical formula is C2H4O.

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 resulting 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

The 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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