First-Order Reaction has rate constant of 7.5 x 10^-3, Time required for reaction to be 60% complete?

Question

I solved the problem and got 68seconds...is this right?

I used Ln [1]/[.6] = -kt for the formula

Answer 1

First of all you don't specify the units of k.
Apparently it is sec^-1

C=Coe^-kt=>
C/Co= e^-kt =>
ln(Co/C) =kt

In this equation C is the remaining amount. If the reaction is 60% complete than 40% remains and C=0.4Co.
thus
t =ln(1/0.4) /(7.5*10^-3) =122.2 sec

Answer 2

extremely... you may use the main objective of what you have LEFT. So one hundred-75=25%. Sooo.. ln(25/one hundred) = - (3.00 x 10^-3)( t ) This expression provides you with the your best option answer. the only above is misguided.