[MilkyCar]

I'm stuck on doing this assignment problem:

A certain first-order reaction is 40% complete in 35 seconds. What are the values of the rate constant and the half-life for this process?

A) k = ? s^-1

B) t1/2 = ? s


So I did ln[A] = -kt + ln[A]0.

Since 40% of the reaction was complete, I figured 60% of it remains. (A = 0.60)

A) So I plugged the values in as:
ln 0.60 = -k x 35 + ln 1
 
And then:
-0.511 = -k x 35
k = 14.6 x 10^-3 s^-1

I got 14.6 x 10^-3 s^-1 as the rate constant since I divided both sides by 35.

B) I took the answer from the first step and did:
t1/2 = (ln 2) / k
t1/2 = 0.693/14.6 x 10^-3
t1/2 = 47.5 s

I got 47.5 seconds as the half-life. I think my calculations must've been off or I'm missing something. Can you please let me know what I should fix?

[mjc123]

What makes you think your answers are wrong?