lemon14 Posts: 143, Reputation: 9 Junior Member #1 Sep 25, 2010, 11:18 PM
molecular formula
after the combustion of 0.4 moles of organic substance results 32.4 g H2O, 4.756 L N2 at 290 K and 1 atm and a quantity of CO2 contained in 240 g solution of NaOH, concentration 40%. Which is the molecular formula of the organic substance?
a) C3H9N
b) C6H6N
c) C3H7N

what I did:
PV = nRT
1*4.756=n*0.082*290
n=0.2 moles of N2

18 g H2O... 1 mole of H
32.4 g H2O... x moles of H
x=1.8 moles of H but I don't think it is correct because the number of moles of N2, H and C should equal 0.4 and here the number of moles of H is bigger than 0.4

I also tried to go back, from the answers to the beginning of the problem, but I got only bad results. Please tell me where I am wrong and how to get the number of the moles of C. if I found these three numbers I could solve the rest of the problem.

ps: the answer in my book is a) C3H9N

thank you and sorry for my mistakes (I am not english)
 Unknown008 Posts: 8,076, Reputation: 723 Uber Member #2 Sep 25, 2010, 11:58 PM

It's okay. The number of moles of nitrogen is good.

The number of moles of hydrogen, however is not, but not because there are more moles of hydrogen.

Note that one mole of $C_3H_9N$ contains 3 moles of C, 9 moles of H and one mole of N.

18 g of H2O --> 2 moles of H

Can you complete this now? :)

For carbon now.

When you burn the substance, you get:

$C_xH_yN_z + O_2 \rightarrow xCO_2 + \frac{y}{2}H_2O + \frac{z}{2}N_2$

according to what you were told.

For the carbon dioxide part, I don't agree with the question, they formulated it in a bad way (or details are absent). 40% of 240 g is 96 g.

So, there are 96 grams of NaHCO3
Find the number of moles of C present in this.

Now, 0.4 mole of the substance contains each of these amounts of moles of each substance. Work it out for 1 mole of the substance.

I hope this helps! :)
 Unknown008 Posts: 8,076, Reputation: 723 Uber Member #3 Sep 26, 2010, 02:30 AM

In one molecule of H2O, you have 2 atoms of hydrogen and 1 atom of oxygen.

Hence, in 1 mole of water, you have 2 moles of hydrogen and 1 mole of oxygen.

I think you have some problem understanding the mole concept.

Having one mole of a substance doesn't mean that you cannot have more than one mole of atoms of an element in that substance.

It's just like a bag.

I tell yo have have three identical bags containing the same number and type of books.

Let's say, in one bag, there are 3 red books, 2 blue books and 1 green book.

In all, I have 9 red books, 6 blue books and 3 green books, which are more in number than the bags themselves!

Now, you can replace bags by moles, and books by atoms.

It is better now? :)

PS - Could you use the answer box instead of the comment button, thanks!
 lemon14 Posts: 143, Reputation: 9 Junior Member #4 Sep 26, 2010, 02:59 AM
Yes, it is much better, thanks! :)

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