$$Molarity of acid = \frac{Molarity of NaOH \times Volume of NaOH (in liters)}{Volume of acid (in liters)}$$
Substituting the given values into the formula:
$$Molarity of acid = \frac{0.2250 M \times 20.5 ml}{40.5 ml}= 0.1114 M$$
Therefore, the molarity of the unknown acid is 0.1114 M.
