[Scholar98]

21.   A first-order reaction is 45% complete at the end of 35 minutes.  What is the length of the half-life of this reaction?
   a)   41 min
   b)   39 min
   c)   30. min
   d)   27 min
ne of these
   ANS:   a)   41 min   PAGE: 12.4
 
How do you solve this. I think you have to use the integrated rate law but I'm not sure how.

[mjc123]

The integrated rate law gives you concentration as a function of time. Rearrange to give time as a function of conc., and insert the data given to find the rate constant. Then calculate the time for 50% reaction.

[Scholar98]

21.   A first-order reaction is 45% complete at the end of 35 minutes.  What is the length of the half-life of this reaction?
   a)   41 min
   b)   39 min
   c)   30. min
   d)   27 min
ne of these
   ANS:   a)   41 min   PAGE: 12.4
 
How do you solve this. I think you have to use the integrated rate law but I'm not sure how.

ln(55)-ln(100)/(35)=41

[mjc123]

ln(55)-ln(100)/(35)=41

That's not correct. Even if you meant {ln(55)-ln(100)}/35, that's not 41 either.
{ln(55)-ln(100)}/35 = -k
Once you know k, you can work out the half-life.

[Scholar98]

.693/0.01708=41

[mjc123]

That's it.