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### Topic: Half-Life  (Read 6322 times)

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#### Scholar98

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##### Half-Life
« 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.

#### mjc123

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##### Re: Half-Life
« Reply #1 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.

#### Scholar98

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##### Re: Half-Life
« Reply #2 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
« Last Edit: January 11, 2015, 09:14:52 PM by Scholar98 »

#### mjc123

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##### Re: Half-Life
« Reply #3 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.

#### Scholar98

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##### Re: Half-Life
« Reply #4 on: January 13, 2015, 11:32:13 PM »
.693/0.01708=41

#### mjc123

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##### Re: Half-Life
« Reply #5 on: January 14, 2015, 04:36:30 AM »
That's it.