Obtaining Empirical and Molecular Formulas from Combustion Data
Empirical and molecular formulas for compounds that contain only carbon and hydrogen (CaHb) or carbon, hydrogen, and oxygen (CaHbOc) can be determined with a process called combustion analysis. The steps for this procedure are:
To illustrate how empirical and molecular formulas can be determined from data derived from combustion analysis, lets consider a substance called trioxane. Formaldehyde, CH2O, is unstable as a pure gas, readily forming a mixture of a substance called trioxane and a polymer called paraformaldehyde. That is why formaldehyde is dissolved in a solvent, like water, before it is sold and used. The molecular formula of trioxane, which contains carbon, hydrogen, and oxygen, can be determined using the data from two different experiments. In the first experiment, 17.471 g of trioxane is burned in the apparatus shown above, and 10.477 g H2O and 25.612 g CO2 are formed. In the second experiment, the molecular mass of trioxane is found to be 90.079.
We can get the molecular formula of a compound from its empirical formula and its molecular mass. (See the text for a reminder of how this is done.) To get the empirical formula, we need to determine the mass in grams of the carbon, hydrogen, and oxygen in 17.471 g of trioxane. Thus, we need to perform these general steps.
Because we assume that all the carbon in trioxane has reacted to form in CO2, we can find the mass of carbon in 17.471 g trioxane by calculating the mass of carbon in 25.612 g CO2.
Because we assume that all of the hydrogen in trioxane has reacted to form H2O, we can find the mass of hydrogen in 17.471 g trioxane by calculating the mass of hydrogen in 10.477 g H2O.
Because trioxane contains only carbon, hydrogen, and oxygen, we can calculate the mass of oxygen by subtracting the masses of carbon and hydrogen from the total mass of trioxane.
We now calculate the empirical formula.
The empirical formula is CH2O, which can be used to calculate the molecular formula.
Empirical formula mass = 1(12.011) + 2(1.00794) + 1(15.9994)
molecular formula C3H6O3