[burntcookie]

The reaction
2 A ---> B + E
is second order (in A) with rate constant 0.0007 L mol–1 s–1. At infinite time [A] = 0. How long will it take a solution with [A] = 85.6 M to decompose such that [A] = 17.4 M? Give your answer in seconds.

so i used 1/[A] - 1/[A0] = kt

t =  (1/[85.6] - 1/[17.4])/0.0007 = 65.4 s

but this answer is wrong. Ideas, anyone?

[Corribus]

How do you know it is wrong? 

[burntcookie]

It's an online quiz for marks, and it says it's wrong.

[Corribus]

Is it part of a larger problem, or that's it?

I got the same answer you did, and I did it blind - i.e., didn't look at your answer first and derived the rate expression without looking at yours.  So either the problem is wrong, or we're both incorrect for the same reason.  I'm a little rusty at deriving rate expressions, though, so I might have missed something obvious...

[Borek]

Perhaps the system expects different number of significant digits?

[Big-Daddy]

The reaction
2 A ---> B + E
is second order (in A) with rate constant 0.0007 L mol–1 s–1. At infinite time [A] = 0. How long will it take a solution with [A] = 85.6 M to decompose such that [A] = 17.4 M? Give your answer in seconds.

so i used 1/[A] - 1/[A0] = kt

t =  (1/[85.6] - 1/[17.4])/0.0007 = 65.4 s

but this answer is wrong. Ideas, anyone?

If you're sure about all the data you gave and the rate constant refers to loss of A (rather than, for instance, gain of B or gain of E, both of which will have the same rate constant, which will be -2*rate constant of A) then your answer is correct to 3 significant figures.