Here’s a video: Start at the 3:56 minute mark.

http://www.brightstorm.com/science/chemistry/chemical-reactions/empirical-formula-molecular-formula/

To Determine an Empirical Formula (a chemical formula that has been reduced to its lowest ratio of subscripts):

1. Assume you have 100g of the substance and convert the % into grams. (In reality, change the % sign to g.)

2. Convert each substance from grams into moles (divide by the atomic mass.)

3. Determine which answer in step 2 is the smallest in magnitude, and divide all the answers in step 2 by that smallest number.

4. You should get whole numbers (or very close to whole numbers, like 1.9 or 2.1 which can be rounded to 2). Those whole numbers are the subscripts that go in the empirical formula.

5. If you don’t get whole numbers, then do this…

If you get .3, then multiply all the answers in step 3 by the number 3, then those are the subscripts.

If you get .5, then multiple all the answers in step 3 by the number 2, then those are the subscripts.

To determine the Molecular Formula (a true formula):

1. Determine the empirical formula if it isn’t given.

2. Calculate the molar mass of the empirical formula.

3. Divide the mass given in the problem (molecular mass or gram formula mass) by the molar mass of the empirical formula. You will get a whole number or really close to a whole number, so round to a whole number.

4. Multiply all the subscripts in the empirical formula by the answer in step 4. Then you will have the molecular formula.