The correct option is Volume percentage (v/v).
Explanation
Concentration units quantify the amount of solute present in a given amount of solvent or solution. The selection of a specific unit often depends on the physical state (solid, liquid, or gas) of the components involved.
- Volume percentage (v/v): This unit is defined as the volume of the solute divided by the total volume of the solution, multiplied by 100. It is the most convenient and commonly employed unit for liquid-liquid mixtures because liquids are easily measured by volume. For instance, commercial products like alcoholic beverages and antifreeze solutions (ethylene glycol in water) express concentration in % v/v.
- Mass percentage (w/w): This unit is typically used for industrial chemical applications or when the solute is a solid dissolved in a liquid.
- Molality and Mole Fraction: These units are primarily used in scientific calculations involving colligative properties (such as boiling point elevation) or vapor pressure, rather than for the general description of liquid mixtures.
Key Takeaway: Volume percentage (v/v) is the standard practical unit for expressing the concentration of solutions where both the solute and the solvent are liquids.