### 7B.7

Posted:

**Fri Mar 13, 2020 7:42 pm**The question says Substance A decomposes in a first order reaction and its half life is 355s. How much time must be elapsed for the concentration of A to decrease to a) one-eight of its initial concentration; b) one fourth of its initial concentration; c) 15% of its initial concentration ; d) one-ninth of its initial concentration?

The answer to b is [A]/[A]0=1/4= (1/2)^2, so the time elapsed 2 half lives. t= 2(355s)=710 s

The answer to d is t=ln([A]0/1/9[A]0)/k= ln9/1.95x10^-3*s^-1=1.1x10^3s

What is the difference between calculating the time elapsed in method b versus in method d? Why can't we use the same method of b on problem d?

The answer to b is [A]/[A]0=1/4= (1/2)^2, so the time elapsed 2 half lives. t= 2(355s)=710 s

The answer to d is t=ln([A]0/1/9[A]0)/k= ln9/1.95x10^-3*s^-1=1.1x10^3s

What is the difference between calculating the time elapsed in method b versus in method d? Why can't we use the same method of b on problem d?