Final answer:
To make a 1000 mL isotonic saline solution, you need to add 9 grams of NaCl, as an isotonic solution usually contains 0.9% NaCl by mass/volume.
Step-by-step explanation:
To determine how many grams of NaCl must be added to 1000 mL of sterile water to make it isotonic, we need to know the isotonic concentration of NaCl, which is typically around 0.9% mass/volume. This means that there are 0.9 grams of NaCl in every 100 mL of solution. So, for 1000 mL (1 L) of solution, we would need ten times that amount, or 9 grams of NaCl.
It is important not to confuse this with the preparation of a molar solution or a saturated solution where the amounts would be different. For example, preparing a 1.00 M solution of NaCl would require 58.44 grams of the substance, which is distinctly different from preparing an isotonic solution.