Need some help with these 3.

1. 15.0gK2Cr2O7 are dissolved into 250mL water. 625mL of a 90g/LHBr solution is mixed with this solution to start the following reaction. What is the limiting reactant? 14HBr+1K2Cr2O73Br2+2KBr+2CrCl3+7H2O 2. A pollutant degrades by first order kinetics. 67 days are required for the concentration of the pollutant to degrade from 55ppm to 35ppm. How much time is required for the pollutant to degrade an additional 20ppm (i.e., from 35ppm to 15ppm )? 3. The following is a second order irreversible reaction: A+A2B The rate constant k1=0.35L/molmin, and the initial concentration of A is 1.2mol/L. Find: a. The time at which the fractional conversion of A is 75%. b. The concentration of A at the time found in part a. c. The concentration of B at the time found in part b