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### Homework Problem M.19 Help

Posted: **Wed Sep 28, 2016 2:29 pm**

by **Jenny_Thompson_3I**

For problem M.19 I am having trouble figuring out where to start due to the fact that it only gives the amounts of carbon dioxide, water, and nitrogen for the products. Are we to just assume that caffeine is composed of carbon, oxygen, hydrogen, and nitrogen and then work backwards using the masses from the products? Any help would be greatly appreciated.

### Re: Homework Problem M.19 Help

Posted: **Wed Sep 28, 2016 6:13 pm**

by **Chem_Mod**

Hi Jenny,

Please write the full question out, as the guidelines for the forum states to do.

### Re: Homework Problem M.19 Help

Posted: **Wed Sep 28, 2016 6:23 pm**

by **Megan Yabumoto 3K**

Yes, since it is a combustion reaction and the products listed are CO2, H2O, and N2, the components of caffeine are C,H,N, and O.

In order to determine the amount of oxygen present in caffeine, you will have to determine the masses of the other elements present and subtract that from the total mass of caffeine given. In order to do so, you must derive the specific elements from the form given as products.

To do so you will have to multiply the 0.682 g CO2 given as a product by (1 mol C/ 44.01 g/mol CO2) to determine the amount of moles of C. Repeat that step for hydrogen by multiplying 0.174 g H2O by (2 mol H/ 18.02 g/mol H2O) and for nitrogen, multiply 0.110 g N2 by (2 mol N/ 28.013 g/mol N2) to determine the amount of moles of the element needed.

Use these moles and determine the amount in grams of each. Add up the amount of grams and subtract that number by the 0.376 g of caffeine given in order to determine the amount in grams of oxygen.

Once you have the masses of each element, proceed to use the steps to determine the empirical and molecular formulas.

I hope this helps!

### Re: Homework Problem M.19 Help

Posted: **Thu Sep 29, 2016 10:58 am**

by **Jenny_Thompson_3I**

Thank you Megan! That was extremely helpful and I though that was the path I should go on so thank you for confirming.

### Re: Homework Problem M.19 Help

Posted: **Thu Oct 05, 2017 5:45 pm**

by **venning-1J**

how do you know there are 2 mol of N and H?