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What is Molality?

Molality is another way of expressing the concentration of a solution. It can be defined as the number of moles of a solute dissolved in 1 kilogram of the solvent.

The symbol for molality is m (small m in italics) or –m (small m with a hyphen). This is to differentiate molality from molarity which is designated by capital letter M; and to different it from mass, which is symbolized with small letter m.

Mathematically stated:

Molality = number of moles/weight of solvent (kg)

The S.I. Unit of molality is therefore, mol/kg

The preparation of a solution of a given molality involves weighing both solute and solvent and getting their masses. But in the case of molarity, the volume of the solution is measured, which leaves room for variations in density as a result of the ambient condition of temperature and pressure.

This means that it is advantageous to work with molality which deals with mass because in chemical composition, the mass of a known pure substance is more important than its volume since volumes can alter under the effect of temperature and pressure while the mass remain unchanged.

Also, chemical reactions take place in proportion of mass, and not volume. For being mass-based, molality can easily be converted into a mass ration or mass fraction, “w” ration.

Both molality and molarity are nearly the same for weak aqueous solutions. For example, 1 kg of water occupies 1 liter (or dm³) of volume at room temperature and the volume is not significantly affected by small amount of solutes.

Question: If 300 g of sugar, C₁₂H₂₂O₁₁, are dissolved in 1500 g of water, what is the molality of the solution?

Solution:

Molality = number of moles/weight of solvent (kg)

Number of moles of sugar = conc. (g)/molar mass

Molar mass of C₁₂H₂₂O₁₁ = 12x12 + 1x22 + 16x11

= 144 + 22 + 176 = 342 g/mol

Number of moles of sugar = 300/342 = 0.877 mols

Therefore,

Molality = 0.877 mols/ 1.5 kg = 0.585 m