Which statement correctly defines molarity and molality?

• A. Molarity (M) = moles solute per liter of solution; Molality (m) = moles solute per kilogram of solvent
• B. Molarity equals moles per kilogram of solvent; Molality equals moles per liter of solution
• C. Molarity equals mass of solute per liter of solution; Molality equals mass of solvent per kilogram of solution
• D. Molarity equals moles solute per liter of solvent; Molality equals moles solute per liter of solution

Answer: (A)

Concentration can be expressed in different ways, and the key idea here is what the denominator is for each measure. Molarity uses the volume of the solution, specifically the number of moles of solute per liter of the entire solution. Molality uses the mass of the solvent, specifically the number of moles of solute per kilogram of solvent. This distinction matters because volume changes with temperature and mixing, while mass does not.

So the correct statement is that molarity is moles per liter of solution and molality is moles per kilogram of solvent. The other ideas mix up these denominators or swap in mass of solute or liters of solvent, which don’t align with the standard definitions.