In a first-order reaction with only one reagent, the reaction was started with a concentration of reactant equal to 0.0800 M. After exactly two hours, the concentration had fallen to 0.0400 M. What is the molarity after exactly four hours?

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In a first-order reaction, the concentration of the reactant decreases exponentially over time. Since the concentration halves every two hours (from 0.0800 M to 0.0400 M), after another two hours (four hours total), it will halve again to 0.0200 M.

In a First-Order Reaction with Only One Reagent, the Reaction