I have worked this problem out so many different ways and i amnot getting the ri

Question

I have worked this problem out so many different ways and i amnot getting the right answer. the right answer is 124, but i haveno idea how they solved it.
The reaction                                               2 Q J is known to have a rate constant of 0.00302 s-1. If webegin a reaction with 0.65 mol/L Q, how long (s) will be requiredto decrease the concentration of Q by 31.28% from its initialvalue?
I have worked this problem out so many different ways and i amnot getting the right answer. the right answer is 124, but i haveno idea how they solved it.
The reaction                                               2 Q J

Explanation / Answer

rate = k*[Q]a rate has units of Molarity/second [Q] has units of molarity k has units of 1/second a = 1, because then dimensions on right and let equal -d[Q]/dt = rate = k*[Q] d[Q]/[Q] = -k*dt ln[Q] - ln[Q0] = -kt ln[Q/Q0] = -kt If decrease by 31.28 %, there is 100 - 31.28 = 68.72% left Q = 0.6872*Q0 Q/Q0 = 0.6872 ln(0.6872) = -0.375129908 = -0.00302 s-1 *t t = 124.215201 second ~ 124 seconds

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