Which method to choose?

For the first one if concentration was a roughly interval variable I would use OLS. Don't make it harder than you have to.

I admire your researchers commitment to science in the 2nd one :p I am not sure if counts are acceptable as interval data. However if you have enough distinct levels they should be. Again I would try OLS.

I dont have any suggestions for the third one, I am not sure exactly what the question is here:)
Thanks you soo much for looking at this!! What is OLS? haha...sorry i am really ****

we've learned the parametric tests, nonparametric tests, anova, linear regression...etc