How is molarity defined?

• A. Moles per liter of solution
• B. Moles per kilogram of solvent
• C. Moles per liter of solvent
• D. Grams per liter

Answer: (A)

Molarity is the amount of solute in moles per liter of solution. This means you divide the number of moles of the dissolved substance by the total volume of the solution, measured in liters. It’s the standard way to express concentration because it directly relates to how much solute is present in a given amount of solution, which is why dilution and reaction calculations use this unit.

This differs from other possible expressions: using kilograms of solvent would give molality, which is about solvent mass, not volume; using liters of solvent would also describe a solvent-based quantity, not the solution’s concentration; and grams per liter is a mass-per-volume concentration, not a mole-per-volume concentration. For example, dissolving 0.5 moles of solute to make exactly one liter of solution yields a molarity of 0.5 M. Note that molarity can change with temperature because the volume of a solution changes with temperature.