Posted by **omar** on Thursday, April 29, 2010 at 1:32pm.

Consider the first order reaction A --> products where 25% of A disappears in 24 seconds. What is the half life of this reaction?

...i thought it would just be doubled but it isn't. i'd appreciate an explanation.

- chemistry -
**DrBob222**, Thursday, April 29, 2010 at 2:24pm
Probably I work these the long way but I do that because then I know I will get it right.

ln(No/N) = kt

I assume a value of 100 for No, and if 25% disappears, that means 75% is left so starting with 100 gives me 75 at the end of 24 seconds.

ln(100/75) = 24*k

solve for k and I get 0.01199 (I know that's too many places but I just leave those numbers in the calculator. Then for the half life

ln(100/50) = 0.01199t_{1/2}

solve for t_{1/2}

Check my math but I obtained 57.8 seconds which would be rounded to 58 sec if the 24 value has just 2 s.f.

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