a substance x decomposes in a second-order reaction. a solution that is initially 1.00 m in x requires 0.50 h for its concentration to decrease to 0.50 m. how much time will it take for a solution of x to decrease in concentration from 1.00 m to 0.25 m? (a) 0.50 h (b) 1.0 h (c) 1.5 h (d) 2.0 h