The molecular weight of NaCl is 58.44 g/mol
So, one mole of NaCl weighs 58.44 g.
A 2.5 M solution is 2.5 moles per liter (Molarity is just the number of moles per liter).
Therefore, 0.5 L would contain 1.25 mol. Hence, you would need 1.25 × 58.44 g = 73 g.
