Molality

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Molality

Molality is the concentration of a solution given in terms of the number of moles of solute in 1 kilogram of solvent.

The molality of a solution is given the symbol m. The molality of a solution can be found by dividing the number of moles of solute by the mass of solvent in kilograms.

It is important to note the distinction between molality and molarity. Molality is a concentration expressed in terms of the mass of solvent, whereas molarity is given in terms of the volume of the solution. In some instances the molality and molarity may in fact have the same numerical value, but it should not be assumed that this is always the case.

If 1kg of solvent has two moles of solute dissolved in it than it is a 2 molal solution. This is generally abbreviated to 2 m. A 1msolution has 1 mole of solute dissolved in 1 kg of solvent and from this it can be seen that the molal unit is an abbreviation for moles per kilogram. One major difference between the molality and molarity is that the molality does not vary with temperature as mass is a constant measure. Molarity can alter with temperature, as it is a function of the volume of the solution, and the volume of solution is affected by temperature (due to expansion and contraction). Molality is a temperature-independent term and molarity is a temperature-dependant term.

When water is the solvent, the molality and molarity of a dilute solution are essentially equal because 1 kg of solvent is nearly the same as 1 kg of solution and, because of the density of water, 1kg of a dilute solution will be very close to 1 L.

Molality used to be called weight molarity which was given the symbol m_w. This term may still be encountered in some older text books.

In dilute solutions where the number of moles of solvent is much greater than the number of moles of solute the following calculation can be used. The molality of a solution is equivalent to 1,000 times the mole fraction concentration of the solute divided by the molecular weight of the solvent.

The ionic strength of an electrolytic solution (one that will conduct electricity) is one half of the sum of all the squares of the charge on the ionic species under consideration, multiplied by the molality of the solution. If discussing a mixture, the summation is carried out for all of the electrolytes in the solution.