molal

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).

This is also known as the molal freezing-point depression constant or simply the freezing-point constant.

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:

It increases to a value of 6.91 for 70 molal acid and then decreases to a value of 0.92 for 1000 molal H2SO4.

The final form of the equation gives the mean molal activity coefficient, , of an electrolyte which dissociates into ions having charges z and z as a function of ionic strength, I.

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.

However, following the SI system of units, the National Institute of Standards and Technology, the United States authority on measurement, considers the term "molal" and the unit symbol "m" to be obsolete, and suggests mol/kg or a related unit of the SI.