prepare 500ml of a 1.25mol/l solution of sodium hydroxide

Answer 1

You need how many mols? That's M x L = 1.25 M x 0.500 = ?
Weigh that much solid NaOH, transfer quantitatively to a 500 mL volumetric flask, add some distilled water, swirl to dissolve all of the solid, cool (it will get hot), add enough distilled water to make to the mark on the volumetric flask, stopper, mix thoroughly, label. done.