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September 30, 2014

September 30, 2014

Posted by **star** on Monday, May 28, 2012 at 10:38am.

a)10 mins

b) 20 mins

c) 40 mins

d)80 mins

e) 160 mins

- chemistry -
**DrBob222**, Monday, May 28, 2012 at 10:43amln(No/N) = kt.

Call No any convenient number, for example, 100, then N = 25, you know t, solve for k.

Then k = 0.693/t_{1/2}

Solve for t_{1/2}

- chemistry -
**star**, Monday, May 28, 2012 at 11:07amstil i didnt get the ryt ans hw to calculate t1/2. Wats the ans ?

- chemistry -
**DrBob222**, Monday, May 28, 2012 at 11:38amWhat answer did did you get. Post your work and I'll find the error.

- chemistry -
**star**, Monday, May 28, 2012 at 12:35pmm doin as k=0.693/40 and m just gettin an different ans

- chemistry -
**DrBob222**, Monday, May 28, 2012 at 12:47pmYou 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 1:28pmm 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 1:41pmk 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 t

_{1/2}. Nor is 2 the half life. The half life (t_{1/2}) is what you are looking for. I don't think your problem is chemistry; I think it is math.

So k = 0.693/t_{1/2}

0.03466 = 0.693/t_{1/2}. Then

t_{1/2}= 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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