To determine the volume of a 2.5 M stock solution of acetic acid (HC2H3O2) required to prepare a given volume of a diluted solution, we can use the formula:

M1 * V1 = M2 * V2

Where:

M1 is the concentration of the stock solution (2.5 M)

V1 is the volume of the stock solution to be used (what we are solving for)

M2 is the desired concentration of the diluted solution

V2 is the desired volume of the diluted solution

Let's assume we want to prepare 100 mL of a 0.5 M acetic acid solution,

Now using the formula:

(2.5 M) * V1 = (0.5 M) * (100 mL)

Solving for V1:

V1 = (0.5 M) * (100 mL) / (2.5 M) = 20 mL

Therefore, to prepare 100 mL of a 0.5 M acetic acid solution, we will need to take 20 mL of the 2.5 M stock solution.