Answer: The initial concentration of hydrogen peroxide at the given temperature is 0.399 M
Step-by-step explanation:
Decomposition of hydrogen peroxide is following first order kinetics.
Rate law expression for first order kinetics is given by the equation:
![k=(2.303)/(t)\log([A_o])/([A])](https://img.qammunity.org/2020/formulas/chemistry/college/67w3lufh8bppkbbcp1xaut7f9029vt6k0l.png)
where,
k = rate constant =

t = time taken for decay process = 855 s
= initial amount of the reactant = ?
[A] = amount left after decay process = 0.321 M
Putting values in above equation, we get:
![2.54* 10^(-4)s^(-1)=(2.303)/(855s)\log ([A_o])/(0.321)](https://img.qammunity.org/2020/formulas/chemistry/college/5l7lydpei08k4s1q6072o6dyk3anxtg0b9.png)
![[A_o]=0.399M](https://img.qammunity.org/2020/formulas/chemistry/college/nsi3hg1lylqrqwbpr6294xalfwcfqjfwhr.png)
Hence, the initial concentration of hydrogen peroxide at the given temperature is 0.399 M