Posted by omar on .
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,
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.