Calculate the half-life (in s) of a first-order reaction if the concentration of the reactant is 0.0837M13.4 s after the reaction starts and is 0.0217M95.9s after the reaction starts. Answer: Suppose the half-life is 25.0s for a first order reaction and the reactant concentration is 0.0414M23.5s after the reaction starts. How many seconds after the start of the reaction does it take for the reactant concentration to decrease to 0.0125M