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How can I calculate the empirical formula of a hydrocarbon?

You must be given the percentages, masses, or number of moles of both Carbon and Hydrogen in the problem.

The basic steps are: % --> Mass --> Moles --> Whole Number

Percent: Assume that the total percentage is out of 100 grams. Then the % = Mass Example: The percentage of Carbon is 56%, so the mass is assumed to be 56 grams (This will always work out in the end)

Mass: Now that we have the mass, convert it to moles. Use the Molar Mass (##g/(mol)##) Divide the mass by the molar mass to get the moles.

Moles: Since the molecular formula of any molecule is based off of whole-number ratios of moles, we must get the moles to a whole number

The easiest way to do this:

  1. Divide the number of moles by the smallest mole number
  2. Multiply all of numbers by the SAME number until they are all close to whole numbers (Usually you will get something like 1.9999, so round that up to 2)

Finally, write the hydrocarbon's empirical formula with the number of moles. The Hydrogen always comes first. Your answer should look something like this: ##CH_4## or ##C_2H_4##

Try it: A hydrogen carbon is made of 25% Hydrogen 75% Carbon. Find the Empirical Formula.

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