Why is it that molality is always larger than the molarity?

Answer 1

Molarity = moles of solute per liter of solution
Molality = moles of solute per kilogram of solvent

When you dissolve most solutes in water, the volume of the solution is slightly greater than the volume of the solvent. So, if you take a mole of sodium chloride and dissolve it in 1 kg of water (1 liter) you'll have a little more than a liter of solution. So, the molality will be 1.0, but the molarity will be a little less.

Answer 2

As you may or may not know, these are the definitions:

Molarity: moles solute / liters solution
Molality: moles solute / kg solvent

If one were to use one mole of NaCl as an example, one would see the following:

Molarity: 1 mol NaCl / 1 L Water + additional volume from added NaCl

This value is less then one.

Molality: 1 mol NaCl / 1 kg Water (which is equal to one liter of Water)

This value is one.

Therefore, the molality is always greater than the molarity because solute and solvent are independent of each other. With a smaller denominator (the solvent in Molality vs. the SOLUTION in Molarity), the value will always be greater for Molality then Molarity, when comparing the values for the same solution.