HW Problem: M19

[Ananta3G]

A stimulant in coffee and tea is caffeine, a substance of molar mass 194 g/mol. When 0.376 g of caffeine was burned, 0.682 g of carbon dioxide, 0.174 g of water, and 0.110 g of nitrogen were formed. Determine the empirical and molecular formulas of caffeine, and write the equation for its combustion.

Please help!

[kevinchang_4I]

This problem is a tough one. Luckily my TA did it in discussion. To find the empirical formula of caffeine you must first the moles of C, H, and N from your products. Remember the subscripts in your conversions! Leave out the O2 for now because there is O2 in the reactants. From there, find out how many grams of each you got from your mole calculations. There should be a difference between that value and the initial caffeine. The difference is the O2 used in the reaction. Now find the moles of oxygen from the grams of difference between the products and reactants. Then, convert those grams to moles of oxygen. Finally divide all the moles you found for the products by the moles of O you have and you should have the empirical formula. This question was super hard I hope my instructions made sense and good luck!

[Paul Hage 2G]

To solve this problem, you would have to use the mass of carbon dioxide to find the moles of C (convert to moles of CO2 using molar mass and then use molar ratio of C in CO2 to find the moles of C). You would then use the mass of H2O to find the moles of H (0.174g H2O/18.02g x mol^-1 H2O to find the moles of H20 and then multiply by 2 because there are 2 moles of H in H2O). Next, you would use the mass of N2 to find the moles of N. After you find the moles of N, H, and C, you would convert each of them to grams. Next, add the grams of C, H, and N and subtract that value from 0.376g to find the mass of O in caffeine. Then you would convert the mass of O to moles. Now that you have the moles of C, H, N, and O, you would divide by the smallest number of moles to find the empirical formula. The molar mass of the empirical formula will be around 97 g/mol, so you would end up having to multiply the empirical formula by 2 to find the molecular formula. Hope that helps.

[Ananta3G]

Yes! Both replies were so helpful, thank you so much. I can assume the product with nitrogen in it is N2, right? because it is a gas?

[Claire Stoecklein 1E]

Hydrogen, Oxygen, Nitrogen, Chlorine, Bromine, Iodine, and Fluorine never occur alone. You can always assume these are H2, N2, etc.