Confusing Half-life problem

[jjkim1232]

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I came across this problem in one of my practice tests and had some trouble figuring it out.

Problem:

If it is determined that the half-life for the first-order decomposition of N2O5 in CCl4 is 6.6x10 -4 sec, according to the reaction:

2 N2O5 --> 4 NO2 + O2

If there is initially 432g of N2O5 (108 g/mol), how many moles of O2 will be produced after two half-lives?


A. 4
B. 2
C. 1
D. 1/2
E. 1/4

 

[fireblast713]

After 2 half lifes 3 moles of N2O5 will have decomposed, so

3 mol N2O5 x (1 mol O2/2 mol N2O5) = 3/2 or 1.5 moles of O2 will have been produced.

 

[jjkim1232]

After 2 half lifes 3 moles of N2O5 will have decomposed, so

3 mol N2O5 x (1 mol O2/2 mol N2O5) = 3/2 or 1.5 moles of O2 will have been produced.

The answer is supposed to be 1/2 moles of O2 produced.

can someone please explain this problem for us?

 

[Donjuan]

The answer is supposed to be 1/2 moles of O2 produced.

can someone please explain this problem for us?

I figured it out, unfortunately I'm being kicked out of my school's library, its closing time, ugh, closes at 8 instead of 11 in the summer, so when I get home, I'll explain it.

 

[fireblast713]

The answer is supposed to be 1/2 moles of O2 produced.

can someone please explain this problem for us?

Wow.. you got me.. I have no idea then.... Wait are you sure that it's not asking after 2 half lifes how many moles of O2 would the remaining N2O5 produce? Because at that point you'd have:

1mol N2O5 remaining x (1mol O2/2mol N2O5) = 1/2mol O2... Looks to me like that might be it.

 

[Donjuan]

Wow.. you got me.. I have no idea then.... Wait are you sure that it's not asking after 2 half lifes how many moles of O2 would the remaining N2O5 produce? Because at that point you'd have:

1mol N2O5 remaining x (1mol O2/2mol N2O5) = 1/2mol O2... Looks to me like that might be it.

Yeah, that's right, I just got home. Remember, its 2 half lives, half the 4 mol two times, then the rest is straight. Hope this helps & Good luck.

- Donjuan

 

[jjkim1232]

That totally makes sense!

The wording of these "word questions" can really be confusing.

By the way...for those that have taken the DAT already:

For GChem, were the balanced reactions given in the problems (ex: Ksp, K, k, combustion, etc.), or did you have to write out your own reaction based on the information given in the problem?


Thanks

 

[fireblast713]

it's a poorly worded problem.

 

[dentalgrl]

hey guys,

does that mean the 6.6 *10-4 thing doesn't matter?