Which rate law should I use?

Question

Which rate law should I use to calculate the rate constant (K)??

the reaction is a first order reaction, should I use:

Ln(A)= -kt + Ln [A]o

Or:

Rate = k [S2O8-][I-] where [S2O82-] and [I-] are the initial reactant concentrations.

Answer 1

It depends on the data that you have. Use Ln(A)= -kt + Ln [A]o if the data you have is reagent concentration vs time, use Rate = k [S2O8-][I-] if the data you have is initial rate vs concentration.