The microtome blade method is used to determine the weight and molal concentration of an interface.

(The molality scale is preferred in thermodynamics because molal concentrations are independent of temperature).

Historically chemists have found it time consuming to calculate natural logs, so they used molal concentrations for dilute solutions as shown below.

The result is that in dilute ideal solutions, the extent of boiling-point elevation is directly proportional to the molal Concentration of the solution according to the equation:

The activity coefficient of the jth ion in solution is written as γ when concentrations are on the molal concentration scale and as y when concentrations are on the molar concentration scale.