Sample Study Sheet: Calculating Molecular Formulas from Combustion Data  

Tip-off You want to calculate the molecular formula of a compound containing only carbon and hydrogen (C_aH_b) or carbon, hydrogen, and oxygen (C_aH_bO_c), and you are given its molecular mass and the masses of CO₂ and H₂O formed in the combustion of a given mass of the compound. 

General Procedure

• Calculate the number of grams of carbon in the compound by calculating the number of grams of carbon in the given amount of CO₂.

• Calculate the number of grams of hydrogen in the compound by calculating the number of grams of hydrogen in the given amount of H₂O.

• If the compound contains oxygen, calculate the number of grams of oxygen in it by subtracting the masses of carbon and hydrogen from the given total mass of compound.

? g O  =  (given) g total  -  (calculated) g C  -  (calculated) g H

• Calculate the empirical formula of the compound from the grams of carbon, hydrogen, and oxygen.

• Calculate the formula mass for the empirical formula and divide the given molecular mass by the empirical formula mass to get n.

• Multiply each of the subscripts in the empirical formula by n to get the molecular formula.

Click here to see an example of this task. 

Click here to see an exercise that will allow you to try this task yourself. 

Return to the Combustion Analysis Page.

  bishopmark@comcast.net

[ Home ] [ Up ] [ Study Sheet ] [ Example ] [ Exercise ]