Final answer:
When preparing a salt solution, a volumetric flask is used to accurately obtain the target solution volume. Starting with less water accounts for the volume increase after solute dissolution to ensure the solution's concentration is precise.
Step-by-step explanation:
The practice of making salt solutions starting with less water than the final desired volume, such as 90 mL of water, is due to the fact that when a solute like sodium chloride (NaCl) is dissolved in water, it will increase the volume of the solution slightly. To accurately prepare a specific molarity, chemists use a volumetric flask that is designed to contain an exact volume after the solute is added.
Therefore, you start with less water, add the solute, and then fill up to the mark to achieve the target volume. This ensures the concentration of the solution is precise. The solute's mass must also be deducted from the total desired solution mass to find the correct amount of water needed, ensuring the final solution has the correct concentration. The example given refers to making a saline solution by dissolving table salt (NaCl) in water where NaCl is the solute and water is the solvent.