## Homework Problem M.19 Help

Jenny_Thompson_3I
Posts: 25
Joined: Wed Sep 21, 2016 2:59 pm

### Homework Problem M.19 Help

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.

Chem_Mod
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### Re: Homework Problem M.19 Help

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

Megan Yabumoto 3K
Posts: 42
Joined: Fri Jul 22, 2016 3:00 am

### Re: Homework Problem M.19 Help

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!

Jenny_Thompson_3I
Posts: 25
Joined: Wed Sep 21, 2016 2:59 pm

### Re: Homework Problem M.19 Help

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

venning-1J
Posts: 12
Joined: Fri Sep 29, 2017 7:04 am

### Re: Homework Problem M.19 Help

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