Question

How to prepare a 250cm3 of 0.5mol/dm3
NaOH solution

Answer 1

How many millimolrd do you want. That's mL x M = 250 cc x 0.5 = 125 millimiles = 0.125 moles.
grams NaOH = moles x molar mass = 0.125 mols x 40 g/mol = ?
Place that many grams in a volumtric flask, add some distilled water, swirl to dissolve, cool (it will get hot or at least warm), add distilled water to the mark on the volumetric flask, stopper, mix thoroughly, label. done. Note carefully that I did NOT (that's right, NOT say) add the NaOH to 250 cc distilled water, mix, label, done. The idea is to add ENOUGH (whatever that amount may be) to the solid NaOH in the flask to make the FINAL VOLUME 250 cc. If you add solid NaOH to 250 cc distilled water the final volume will NOT be (if you can measure it carefully enough) 250 cc.