a solution of weak acid of molarity 0.05 mol/dm3 is titrated against 0.1mol/dm3 NaOH, the pka = 3.67

Question

what volume of acid would u use in ur titration?

Answer 1

The problem doesn't say the weak acid is monoprotic but it implies that it is by listing only one pKa.
molarity x volume = molarity x volume
0.05 x volume = 0.1 x volume
You need one of the volumes if you are to titrate to the end point. If the question is how much acid to use for the titration, I would think around 40 dm^3 of the acid because that will give about 20 cm^3 of the base and you want to keep the reading errors of the buret as low as possible. (The actual reading error is the same whether its 2 dm^3 or 20 dm^3 BUT the percent error is larger for the 2 dm^3 reading.)