A second-order reaction (2A B) with a rate constant of 0.350 M-1s -1 is found to have a half-life of 3.45 s. What was the initial concentration of

How do I know to use the Half life formula versus integrated law? What is the difference in time in each? I used the integrated law first because I thought for t, I could use half life time given and it was wrong. the correct answer is the one that used the half life