What is molarity?

• A. The mass of solute per liter of solution.
• B. The number of particles per liter.
• C. The number of moles of solute per liter of solution.
• D. The volume of solvent per mole of solute.

Answer: (C)

Molarity is a way to express concentration by saying how many moles of solute are present in one liter of solution. The key is that it uses moles for the amount of solute and liters for the volume of the entire solution, not just the solvent or the solution’s mass. This makes molarity a practical bridge between the amount of substance and the volume you work with in the lab.

Because it uses moles, not just a simple count of particles, it connects microscopic quantities to a measurable, scalable unit. If you only counted particles per liter, you’d be skipping the step of converting that count into moles, which is why “number of particles per liter” isn’t the standard molarity unit. Likewise, molarity is not about the volume of solvent per mole of solute, which would be an inverse relationship and refers to a different concept.

For example, dissolving 0.80 moles of solute so that the total solution volume is 2.0 liters gives a molarity of 0.40 M (0.80 mol / 2.0 L). In practice, you can use M = n / V to find either the amount of solute needed (n = M × V) or the volume of solution required (V = n / M) for a given reaction or procedure. This makes molarity a fundamental tool for planning and performing quantitative lab tasks.