Half-life equation for first-order reactions: (the 1/2 in t1/2 is a subscript) t1/2=0.693/k where t1/2 is the half-life in seconds (s) , and k is the rate constant in inverse seconds (s^-1) . Part A What is the half-life of a first-order reaction with a rate constant of 5.50×10^?4(s^-1)? Part B What is the rate constant of a first-order reaction that takes 208 seconds for the reactant concentration to drop to half of its initial value? Part C A certain first-order reaction has a rate constant of 1.20×10^?3 s^-1. How long will it take for the reactant concentration to drop to 1/8 of its initial value?