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Chapter 7.4 - Empirical Formulas

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Chapter 7.4 - Empirical Formulas (Determining Chemical Formulas)

To Determine Empirical Formula from Percent Composition:

1 - Convert the Percentage (%) to Mass (g) - "assume" 100.0 g sample.

* Basically, just change the % to g, and you will be okay

2 - Calculate moles by dividing the grams (from step 1) by the molar mass of the element (found on the periodic table).

3 - Make the simplest ratio possible. From the answers made in step 2, pick the smallest one (the smallest number of moles) and divide all the numbers by it. The smallest answer you should get is "1" - if you get something like 1.5, double everything to get whole numbers!

* These are the subscripts for your Empirical Formula.

To determine Molecular Formula - you will need the Empirical Formula and the molar mass of the compound (will be given to you).

4 - Calculate the Empirical Mass of the Empirical Formula. This is done the same way you figure out the molar mass of a compound.

5 - Divide the Molar Mass (given to you) by the Empirical Mass (from step 4).

6 - Multiply all the subscripts in the Empirical Formula by the answer you found in step 5.

* This is the Molecular Formula of the compound.

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