[Hapster]

A Free-Response AP Exam question that I have been having a little bit of trouble with:


1.)  Combustion of 8.652 grams of a compound containing C, H, O, and N yields 11.088 grams of CO2, 3.780 grams of H2O and 3.864 grams of NO2.

     a)  How many moles of C, H, and N are contained in the sample?  [do you just convert the grams of each of the products (CO2, H2O, and NO2) to moles then compare the percent of each individual atom (C, H, and N) to the moles of the entire compound?]

     b)  How many grams of oxygen are contained in the sample?

     c)  What is the simplest formula of the compound?

     d)  If the molar mass of the compound lies between 200 and 300, what is its molecular formula?

     e)  Write and balance a chemical equation for the combustion of the compound. 





If anyone can go through the entire problem it would be great, but if you just want to answer a part of it, it would be helpful as well.  Please try to include explainations in your answer too, so I can see how you arrived at your conclusion.  Thanks in advance to those who undertake this challenge   

[Borek]

Please read forum rules. You must show your attempt.

[Yggdrasil]

     a)  How many moles of C, H, and N are contained in the sample?  [do you just convert the grams of each of the products (CO2, H2O, and NO2) to moles then compare the percent of each individual atom (C, H, and N) to the moles of the entire compound?]

Basically, yes.

b)  How many grams of oxygen are contained in the sample?

A hint here would be to try to determine the grams of oxygen gas that must have reacted with your compound (recall conservation of mass).


Try answering these first and then see whether you can do the rest.

[Hapster]

A hint here would be to try to determine the grams of oxygen gas that must have reacted with your compound (recall conservation of mass).


Try answering these first and then see whether you can do the rest.

Yeah that's what I was thinking for B, so becuase there is a total of 18.732 g in the products, and the compound has a mass of 8.652, 18.732-8.652= 10.08 g of O.  Ok, now that accounts for the oxygen that reacts out of the air, but what about the oxygen that is present in the compound?  Shouldn't I simply find the moles of carbon in all of the products and then add them all up?

question c.) was really the one I was having trouble with, I have the basic formula down:

C H O N + O2 --> CO2 + H2O + NO2

Now, the trouble I have is just finding the subscripts of each elements in the CHON...  I'm sure it has something to do with comparing the masses, in order to uphold the law of conservation of mass, but I am stuck here.  How would I go about finding the subscripts?  I know I am finding the empirical formula, but only know how to do this when the problem gives the % composition of each element. 

d.) will be easy once I can find the answer to c.), it is just comparing the empirical formula's mass to the actual mass. 

Again, e.) will also be easy once c.) and d.) are found. 

So essentially, I am just in need of help for question c.), and then I think I can handle the rest of the problem alone.   

[Yggdrasil]

On the product side, write down how many moles of C, N, H, and O you have.  The number of moles of C on the product side must be equal to the number of moles of C on the reactant side, the number of moles of N on the products side must be equal to the number of moles of N on the reactant side, etc.  Doing this should help you find an answer to the question.