Calculating the empirical Formula of Organic Compounds!

Yummmm, organic beets!

The empirical formula of an organic compound can be found by COMBUSTING the compound (reacting it with oxygen). The mass of the products can then be measured to find how much of each reactant was present. Recall: The Law of Conservation of Mass: the mass of the reactants will always equal the mass of the products!

Let's try an easy example: 

A 7.30 gram sample of a hydrocarbon is burned to give 23.8 grams of CO₂ and 7.30 grams of H₂O. What is the empirical formula?  

_{Notice that all of the C's and all of the H's went into making the carbon dioxide and water!} 

Step 1: Convert the grams of carbon dioxide and water into moles

Mol CO2 = 23.8g CO2 x (1Mol CO2)/(44.0g CO2)* = 0.541 mol CO2

* This is the molar mass of CO2

Mol H2O =7.30g H2O x (1Mol H2O)/(18.0g H2O)* = 0.406 mol H2O

* This is the molar mass of H2O

After this step, you can conclude that 0.541 moles of CO2 and 0.406 moles of H2O were produced.

Step 2: We want to isolate the carbon in CO2 and the hydrogen in H2O, these are the elements that make up organic compounds.

Mol C = 0.541 mole CO2 x (1 mole C)*/(1 mole CO2) = 0.541 mol
*Number of moles of C in 1 mole of CO2

Mol H = 0.406 mole H2O x (2 mole H)*/(1 mole CO2) = 0.812 mol

*Number of moles of H in 1 mole of H2O

After this step, you can conclude 0.541 moles of C and 0.812 moles of O were in the original organic substance.

Step 3: Recall how to find the empirical mass.

Divide both number of moles by the smallest molar amount (in this case it's C > 0.541)

C> 0.541/0.541 = 1 (Now read this: We have to multiply this because we multiplied 1.5)

H> 0.812/0.541 = 1.5 (Read this first: We have to multiply by 2 to make this into a whole number)

So, the empirical formula would be C2H3

Step 4: Check your answers!

Convert the moles of C and the moles of H to grams - they should add up to 7.30g

0.541 moles C x (12.0g C)/(1 mole C) = 6.49g C

0.812 moles H x (1.0g H)/(1 mole H) = 0.812g H

6.49g C + 0.812g H = 7.30g

If after checking your answer, you find that your masses do not add up, realize there must be a component of oxygen present in the compound. 

The mass of O = Mass of the compound - Mass of C + Mass of H

The mass of oxygen can then be converted to moles. Reapply step 3.

Some easy review for the quiz - http://lhs2.lps.org/staff/sputnam/practice/UnitV_EmpForm.htm

Here is a step by step tutorial on how to find empirical formula and molecular formula or the "real formula."