What is the molarity of an unknown acid if 40.5 ml of the acid can be titrated to an end point by 20.5 ml of 0.2250 M NaOH?

Answer 1

We can't know if we don't know if the acid is monoprotic, diprotic, or triprotic. So which kind of acid is it and which equivalence point did we reach? If it's monoprotic you can use the simple equation of
mL x M = mL x M.

Answer 2

0.114 M