EXAMPLE Obtaining a Molecular Formula from Combustion Data:  

Dianabol is one of the anabolic steroids that has been used by some athletes to increase the size and strength of their muscles. It is similar to the male hormone testosterone. Some studies indicate that the desired effects of the drug are minimal, and the side effects, which include sterility and increased risk of liver cancer and heart disease, keep most people from using it. The molecular formula of Dianabol, which consists of carbon, hydrogen, and oxygen, can be determined using the data from two different experiments. In the first experiment, 14.765 g of Dianabol is burned, and 43.257 g CO2 and 12.395 g H2O are formed. In the second experiment, the molecular mass of Dianabol is found to be 300.44.  What is the molecular formula for Dianabol?


? g O  =  14.765 g total  - 11.805 g C  -  1.3870 g H  =  1.573 g O

We now calculate the empirical formula.

The empirical formula is C10H14O. We use it to calculate the molecular formula:

Empirical formula mass  =  10(12.011)  +  14(1.00794)  +  1(15.9994)  =  150.22

molecular formula         C20H28O2

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