Posted by Darali on Thursday, April 8, 2010 at 7:02am.
Combustion analysis of 0.284 g of a compound containing C, H, and O produces 0.623 g of CO2 and 0.340 g of H2O. Mass spectral analysis shows that the compound has a molar mass around 120 g mol1. What is the compound's:
Empirical Formula: Molecular Formula:

Chemistry  DrBob222, Thursday, April 8, 2010 at 1:57pm
Convert 0.623 g CO2 to g C then moles C.
Convert 0.340 g H2O to g H then moles H.
Add g C to g H and subtract from 0.284 to obtain g O.
Now you want to find the ratio of the MOLES of these three elements to each other in small whole numbers. The easy way to do that is to divide the smallest number by itself which assures you of getting 1.000 for that element. Then divide the other two numbers by the same small number and round to a whole numbers. This will obtain the empirical formula.
To find the molecular formula, add the atomic masses of the elements to find the empirical mass and divide that number into 120, then round to a whole number which I will call x. That is the number of units of empirical formula you have; i.e., (CyHzOw)x.
Post your work if you get stuck.
This sound complicated because there are several steps but its quite straight forward.