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

21. A firstorder reaction is 45% complete at the end of 35 minutes. What is the length of the halflife 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.

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.

21. A firstorder reaction is 45% complete at the end of 35 minutes. What is the length of the halflife 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

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 halflife.

.693/0.01708=41

That's it.