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What is Molality?

A final way to express the concentration of a given solution is by calculating its molality. The molality (m) of a solution is defined as the moles of solute divided by the kilograms of solvent in which solute is dissolved. For instance; A solution that comprises of 1.0 mol of NaCl dissolved into 1.0 kg of water is referred to as a “one-molal” solution of sodium chloride. The symbol for molality is represented in a lower-case m.

Molality Formula

Molality(m)=Number of moles of solute divided by Mass of solvent in kgs.

SI unit of molality = mol kg-1.

Molality (m) = moles of solute/kilograms of solvent = mol/kg

Molality Characteristic

1. Molality is denoted as a property of a solution.

2. It is an intensive property thus it will not differ from sample to sample for a given solution.

3. The number of moles of solute and the mass of solvent are generally not affected by pressure and temperature of system. Henceforth, molality unlike molarity, is not dependent on temperature and pressure.

4. Molality is referred to a non-volumetric concentration such as mole fraction and mass fraction.

Molality vs Molarity

Molarity (M) is commonly defined as the number of moles of solute per liter of solution.

Molarity = moles of solute/liters of solution

Molality (m) is commonly defined as the number of moles of solute per kilogram of solvent.

Molality = moles of solute/kilograms of solvent

Molarity is referred to measurement of the moles in the total volume of the solution, whereas molality is a measurement of the moles in connection to the mass of the solvent.

Molality varies from molarity only in its the denominator. While molarity is expressed in the liters of solution, molality is expressed in the kilograms of solvent.

Concentrations expressed in molality are commonly used to study properties of solutions which are related to vapor pressure and temperature changes. Molality is often used because its value generally does not change when temperature of system is change thus it is an independent quantity. On the other hand, the volume of a solution, is somewhat dependent upon temperature.

Molality and molarity are related in value for dilute aqueous solutions because of the fact that the density of those solutions is fairly close to 1.0 g/mL. This further means that 1.0 L of solution has approximately a mass of 1.0 kg. As the solution becomes more and more concentrated, its density will not even be close to 1.0 g/ml and the molality value will differ from molarity.

How to Calculate Molality? Example 1

What is the molality when 0.830 mol is dissolved in 1.60 L of solvent?

Molality = 0.830 mol / 1.60 kg

The answer is 0.518 m.

How to Calculate Molality? Example 2

Calculate the molality of a solution prepared from 30.22 grams of NaCl in 5.00 kg of water.

Given;

Solute = 30.22 gm of NaCl

Solvent = 5.00 Kg of water

Molar mass of solute that is NaCl = 58.44 gm/mol

number of moles of solute = 30.22 / 58.44

= 0.51 mol

Molality or m = number of moles of solute (n)/ weight of solvent in kg

Thus,

m = 0.51 / 5.00 = 0.102 moles/kg

Molality Citations

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