Generally if the distance between each level is the same, there are many distinct levels (12 or more is a reasonable rule of thumb although formally the answer is infinite) and the levels are ordered you can treat it as interval. I would think concentration, a percent normally, would certainly be considered interval. One possibility if you are unsure is to look at how past literature has treated concentrations.

No one calculates p values manually (well Dason might, but he is strange).

Any decent statistical software will generate a p value. You have to decide what test you are running first. To me, assuming your dependent variable is interval which I suspect it is, then regression or ANOVA seems the most logical. ANOVA is commonly used in medical or biological research - not sure if that is what you are doing.