Why dont you need to know the exact concentration of the NaOH used when finding the acid dissocation constant?

I just finished a AP Chemistry lab and this was one of the questions. We were given and unknown acid and told to find the pKa. We put between .15 and .2 g of this unknown into 50 mL of distilled water and stirred it till it all dissolved then took 25 mL of that solution and put it into a flask, then we dripped NaOH into it until the NaOH stayed pink for a few seconds then we poured the 25 mL + NaOH back into the beaker with the untouched solution and then we used a probe to find the pH.