Question

If a student needs to make exactly 2.5 liters of

Answer 1

260 mL

The concentration of the solution must go from 12.0 M, which is what you have for the stock solution, to 1.25 M.
The dilution factor will thus be
12/1.25= 9.6
This means that the volume of the stock solution used for the dilution was
2.5 L/9.6= 0.26 L
Expressed in milliliters and rounded to two sig figs, the answer will be 260 mL.