What happens to the volume of a reagent if it is more concentrated?

• A. The volume decreases
• B. The volume increases
• C. The volume stays the same
• D. The volume doubles

Answer: (A)

Concentration is the amount of solute in a certain volume. If you keep the amount of solute the same and make the solution more concentrated, you must have less solvent, so the total volume decreases. Since concentration = mass of solute ÷ volume, increasing concentration with the same mass means the volume is reduced (volume = mass ÷ concentration). For example, 5 g of solute in 0.1 L is 50 g/L; if you increase the concentration to 100 g/L with the same 5 g of solute, the volume drops to 0.05 L (50 mL). So the volume decreases.