Posted by star on Monday, May 28, 2012 at 10:38am.
a radioactive material is undergoin nuclear decay. After 40 minutes, 25% of the sample remains. What is the half life of the sample? Pls say the formula how to calculate it
b) 20 mins
c) 40 mins
e) 160 mins
chemistry - DrBob222, Monday, May 28, 2012 at 10:43am
ln(No/N) = kt.
Call No any convenient number, for example, 100, then N = 25, you know t, solve for k.
Then k = 0.693/t1/2
Solve for t1/2
chemistry - star, Monday, May 28, 2012 at 11:07am
stil i didn't get the ryt ans hw to calculate t1/2. Wats the ans ?
chemistry - DrBob222, Monday, May 28, 2012 at 11:38am
What answer did did you get. Post your work and I'll find the error.
chemistry - star, Monday, May 28, 2012 at 12:35pm
m doin as k=0.693/40 and m just gettin an different ans
chemistry - DrBob222, Monday, May 28, 2012 at 12:47pm
You aren't following my instructions; that's why you aren't getting the right answer.
You must find k FIRST before substituting into the half life part.
Show what you have for ln(No/N) = kt. I told you what to substitute for No and N. Substitute 40 for t.
chemistry - star, Monday, May 28, 2012 at 1:28pm
m getting k as 0.03465 m doin k =0.693/40x2 so m getting k as 0.03465
chemistry - DrBob222, Monday, May 28, 2012 at 1:41pm
k is correct. k = 0.03466. The rest of what you are doing is not right. 40 min is the time it took to decrease from 100 to 25. 40 min is not t1/2. Nor is 2 the half life. The half life (t1/2) is what you are looking for. I don't think your problem is chemistry; I think it is math.
So k = 0.693/t1/2
0.03466 = 0.693/t1/2. Then
t1/2 = 0.693/0.03466 = 19.996 minutes which rounds to 20 min. The reason it doesn't come out exactly 20 min is because 0.693 is what I used for ln 2 and it actually is 0.69315.
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