take 58.5 and divide that by 0.5, we get 117 grams of Sodium Iodide per liter
If you are looking for molarity. Then there are 149.88 grams in one mole of Sodium Iodide. We have 0.78 moles of sodium iodide. Divide that by 0.5 liters (also dm3)
0.78 / 0.5 = 1.56 M