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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Scholar98 on January 11, 2015, 02:55:01 PM

Title: Half-Life
Post by: Scholar98 on January 11, 2015, 02:55:01 PM
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.
Title: Re: Half-Life
Post by: mjc123 on January 11, 2015, 05:06:56 PM
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.
Title: Re: Half-Life
Post by: Scholar98 on January 11, 2015, 07:38:57 PM
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
Title: Re: Half-Life
Post by: mjc123 on January 12, 2015, 05:10:01 AM
Quote
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.
Title: Re: Half-Life
Post by: Scholar98 on January 13, 2015, 11:32:13 PM
.693/0.01708=41
Title: Re: Half-Life
Post by: mjc123 on January 14, 2015, 04:36:30 AM
That's it.