Lesson: Percent composition to Molecular Formula

Determining molecular formula from peercent composition.

By the end of this lesson, you should be able to take the percent composition of a compound and determine the compound's empirical and then molecular formula when given the compound's molar or formula mass.

​Let's first review the steps in determining the empirical formula from a given percent composition.

1. if percent composition is given, change the percent into grams (example 27% = 27 g)

2. convert the gram of each element into mole for each elements by dividing by the element's molar mass.

3. find th lowest number of moles and divide that into the moles of each element. Multiply all the elements ratio until you get all whole numbers.

​Now, we will take this to the nest step, which is to find the molecular formula for the compound.

Notice, the only new step in finindg the moelcular formula is to finds the "multiplying factor" that was used to divide to get the empirical formula. To do that, we go to step 4-6.

1. if percent composition is given, change the percent into grams (example 27% = 27 g)

2. convert the gram of each element into mole for each elements by dividing by the element's molar mass.

3. find th lowest number of moles and divide that into the moles of each element. Multiply all the elements ratio until you get all whole numbers.

4. Find the formula mass of the emprical formula.

5. Take the molecular mass given and divide it by the formula mass of the emprical formula to get a multiplier.

6. Use the multiplier and multiply the emprical formula

​Try this.

A sample was analyized and found to contain 40.00% C, 6.71 % H, and 53.29 % O. If the molecular mass of the compound is 180.16 g/mol, what is are the emprical and molecular formula of the compound?
Please show your work. Follow the steps.

​if percent composition is given, change the percent into grams (example 27% = 27 g)

1. convert the gram of each element into mole for each elements by dividing by the element's molar mass.

2. find th lowest number of moles and divide that into the moles of each element. Multiply all the elements ratio until you get all whole numbers.

3. Find the formula mass of the emprical formula.

4. Take the molecular mass given and divide it by the formula mass of the emprical formula to get a multiplier.

5. Use the multiplier and multiply the emprical formula

​Now, Try this.

A sample was analyzed and found to contain 24.74% K, 34.76% Mn and 40.50% O. If the molecular weight of the compound is 158.03 g/mol, what is the emprical and molecular for the compound? Show your work!

Hopefully, now you understand how to go from percent composition to empirical formula and then to molecular formula. Just remember to follow the steps below.

1. if percent composition is given, change the percent into grams (example 27% = 27 g)

2. convert the gram of each element into mole for each elements by dividing by the element's molar mass.

3. find th lowest number of moles and divide that into the moles of each element. Multiply all the elements ratio until you get all whole numbers.

4. Find the formula mass of the emprical formula.

5. Take the molecular mass given and divide it by the formula mass of the emprical formula to get a multiplier.

6. Use the multiplier and multiply the emprical formula