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Empirical Formula
The empirical formula of a substance represents the smallest whole number ratio of the elements in that substance
- ionic compounds-only have an empirical formula
- molecular compounds-have both an empirical and a molecular formula
Steps to finding the empirical formula of a substance:
- Find the mass percent of a certain element in the original compound.
- Assume 100g of the compound and find the number of moles present of each element using their atomic masses.
- Divide each mole value by the smallest of the values.
- If, after rounding, you get a whole number result, then these values will represent the subscripts of the elements in the compound in its empirical form.
- If you do not end up with whole numbers as a result of the division, multipy all of the results by an integer until all of the subscripts turn out to be whole numbers.
Example:
Suppose .1234g of a substance, that is composed only of the elements Carbon, Hydrogen, and Oxygen, is reacted with oxygen to form .1345g CO2 and .1456g H2O. Asume that all of the Carbon is consumed to form CO2 and that all of the Hydrogen is consumed to form H2O. Find the empirical formula of the substance.
- To find the mass percent of Carbon in the original compound:
- First you must determine the molar mass of CO2:
C: 1mol x 12.01g/mol = 12.01g
O: 2mol x 16.00g/mol = 32.00g
Molar Mass of CO2=44.01g/mol
- Next, find the mass percent of carbon in CO2
(Mass of C)/(Total Mass of CO2) = (12.01g C)/(44.01g CO2)