The standard (and oft vindicated) line is that phenethylamines of this sort are very stable, even in solution. You could use ethanol, or distilled water (no chlorine!), or a mixture of the two. The resulting solution will probably be fine for many months, or years, at room temperature, but you could also make aliquots and freeze them (or dissolve in ethanol, and keep the whole vial in the freezer). Avoid freeze-thaw cycles, in any case.
500 micrograms = 0.5 mg. You have 50 mg, so that's 100 times 0.01 ml = 1 ml. 50 mg in 1 ml might be pushing the limits of solubility for DOC (someone else may know more), but in any case, unless you have some micropipettes lying around, measuring out ten microlitres for a 0.5 mg dose (or 40 for 2 mg) is going to be impossible. How are you planning to use it?