Question: how many grams of sodium chloride are required to prepare 500 ml?
Answer: 10 mg. Mass of 500 mL NaCl solution is roughly 500 mg, because water density is about 1 mg/ml (so low 2% NaCl concentration doesn't make significant difference in density). So as NaCl mass is 2% of the solution mass - 0,02*500=10mg.