thermodynamics

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yingao

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N5 + (g) + N5 - (g)--> 2N3(g) + N2 (g)

in a theoretical reaction, a sample N5+N5- is allowed to completely decomposed and the resultant N2 is collected. At STP the volume of the gas is measured to be 112 L. What was the mass of the original sample?

1. 70g
2. 140g
3. 280g
4. 350g

ans: 2

I know the N2 is 5 moles, but what to do after that?

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Aced thermo and heat transfer at PU last year and didnt come across this. However, in chem 1 beat it like a red headed step child. Too lazy to get my notes out now, maybe tomorrow.
 
Remember that at STP one mole of gas has a volume of 22.4L you can then find the number of moles of N2 created from the reaction and then the mass of the reactant
 
N5 + (g) + N5 - (g)--> 2N3(g) + N2 (g)

in a theoretical reaction, a sample N5+N5- is allowed to completely decomposed and the resultant N2 is collected. At STP the volume of the gas is measured to be 112 L. What was the mass of the original sample?

1. 70g
2. 140g
3. 280g
4. 350g

ans: 2

I know the N2 is 5 moles, but what to do after that?


YOu find the moles of N2. that is obviously 5 moles. and then multiply by 28 grams. that should get you mass of N2 in the sample. which is 140 g. As a side note for MCAT, round 112 to 110 and 22.4 to 22. easier to work with and then figure out the answer. it should be close to one of hte answer choice. in this case, you get lucky and you get exact 140 g
 
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YOu find the moles of N2. that is obviously 5 moles. and then multiply by 28 grams. that should get you mass of N2 in the sample. which is 140 g. As a side note for MCAT, round 112 to 110 and 22.4 to 22. easier to work with and then figure out the answer. it should be close to one of hte answer choice. in this case, you get lucky and you get exact 140 g


so the mass of the original sample is N2 not N5?
 
hmmmmm.

That reaction is not balanced. It should be N5(+) + N5(-) ----> 2N3 + 2N2

we have 112L of gas evolved at STP implying we have 5 moles of N2 evolved at STP. For every 2 mol of N2 there is 1 mol of each reactant. I dont know if I am doing this right, but would it work to say we have 5 mol of N2 in this reaction corresponds to 5/2 mol of N5. 5/2 mol(70 g/mol N5) is 170 closest answer is 140g.

I have no idea if this is right. Im tripped by this one to. Just trying to throw ideas out there.
 
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