Hi, I am studying for a midterm and I need help on 2 practice questions from the textbook which I have no idea how to start. The first one is: Calculate the atmospheric pressure at the stratopause. What are the concentrations (mol m-3) of dioxygen and dinitrogen at this altitude? How do these concentrations compare with the corresponding values at sea level? I have no idea how I am able to start this question. Any tips would be greatly appreciated. The second one is: At a particular temperature, the Arrhenius parameters for the reaction: •OH + H2 → H2O + •H Are A = 8 x 10^10 s -1, and Ea = 42 kJ mol-1. Given that the concentration of hydroxyl in the atmosphere is 7 x 10^5 molecules cm-3 and that of H2 is 530 ppbv, calculate the rate of reaction (units of molecule cm-3 s-1) for this process. I have tried this question with other formulas but it just always ends up with a very very large number. Any tips would be greatly appreciated. Thank You.