Question M19: Empirical and Molecular formula of caffeine [ENDORSED]

[rosemarywang4i]

Question M19 asks:

"When 0.376g of caffeine was burned, 0.682g carbon dioxide, 0.174g water, and 0.110g nitrogen were formed. Determine the empirical and molecular formulas of caffeine, and write the equation for its combustion."

I know that when something is combusted, it is reacted with oxygen to produce carbon dioxide and water. However, I'm confused as to how the elements in caffeine are to be isolated from this information. From there, I should be able to calculate the empirical and molecular formulas themselves. Any insight would be appreciated.

Thanks!

[Hai-Lin Yeh 1J]

As you said, when something is combusted, it reacts with oxygen to produce CO2 and water, H2O, and you aren't sure what elements are in caffeine. But, if you write the chemical equation out:

Caffeine (unknown formula) + O2 -> CO2 + H2O + N2. Looking at the chemical equation, the reactants are caffeine and O2, but the products given are CO2, H2O, and N2. The nitrogen, carbon, and hydrogen must have come from somewhere. Definitely not the O2. Therefore, based on the process of elimination, caffeine must contain nitrogen, carbon, hydrogen, and oxygen. Then, you will have to figure out how much O2 was actually from caffeine, but you know that the hydrogen, carbon, and nitrogen definitely came from caffeine.

Hope this helps!

[Albert Duong 4C]

You want to first find mass of each element through stoichiometry using the masses of CO2, H2O, and N2 given. For example, you can find the grams of carbon (C) from grams of CO2 by finding the moles of CO2, and then finding the moles of C (it's the same moles as CO2 since both CO2 and C have only 1 carbon atom), which then you can find the grams of C easily. Find the masses of H and N, and then subtract the total masses of C, H, and N from the mass of caffeine (0.376 g) to find the mass of O. Convert all the elements to moles and then find the empirical and molecular formula (you probably already know how to do that).

[gillianozawa4I]

I found the answer in a similar way. It is easiest to find the mass of elements that appear the least number of times in the equation first. You can isolate the carbon from CO2 by multiplying 1 mol of C/1 mol of CO2, because there is 1 carbon per 1 CO2 (1:1 ratio). It is definitely easiest to save finding the mass of oxygen for last because it appears so many times in the equation. Hope this helps!

[705192887]

By process of elimination, you know that the hydrogen, carbon, and nitrogen must come from the caffeine molecule, since they couldn't have come from O2. After this, solve for the empirical formula using the mass of the products formed, and those masses are the same as the masses in the original caffeine reactant (Law of Conservation of Mass). Hope this helps!!